Abstract
Objectives. to understand the resistance to cyclic and static contact loading of feldspathic
porcelain on dental zirconia (3Y-TZP), in order to understand the partial failure of porcelain
(chipping or cracking).
Methods. Hertzian contact techniques were used to evaluate the appearance of damage as a
ring crack in terms of applied load and number of cycles in air and simulated saliva
Results. Static contact loading showed the presence of stress corrosion cracking in the
porcelain; the environmental crack growth in air was determined from the results of time to
damage under static load. There was also a contribution of fatigue effects due to the
interactions of the crack with the microstructure. From the obtained results, a time to failure
was estimated depending on the materi
al counterpart. Cracking can occur on porcelain
coatings if the contact counterpart is teeth or porcelain in a time interval of a few years,
consistent with clinical studies.
Significance. Contact loading, particularly against teeth or other ceramic materials can be a
significant cause of failure and chipping of feldspathic porcelain on zirconia, especially if the
patient suffers from bruxism. Protection, by e.g. a guard, against repetitive contact against the
porcelain can increase the lifetime of the veneer.
Keywords: Feldspathic dental porcelain, zirconia, Hertzian contact, fracture, fatigue.
1. Introduction
Feldspathic dental porcelains on zirconia are increasingly being used in dental restorations
due to their aesthetic appearance similar to natural teeth as well as biocompatibility, corrosion
resistance and mechanical properties [1-3]. The fracture of dental porcelains and ceramics is a
problem affecting the integrity of the dental components [1]. High masticatory forces may
induce fracture or deformation in the dental restoration that can lead to premature failure [2],
but the majority of failures occur after some time. Studies of zirconia-based restorations
reported fracture, chipping, and/or delamination as the predominant failure modes primarily
in single crowns and bridges, without observing framework failures [4-8].
These failures are attributed to different reasons, such as low fracture toughness, inappropriate
framework support, low cohesiveness, shear forces between the zirconia framework and
veneering porcelain as well as thickness reduction or cracking after occlusal contact in the
oral environment [1,9-11].
Different papers in the literature described veneering porcelain failures after 2 to 5 years of
functioning with rates ranging from 0 to 30% depending on the material and service time
[6,12-15]. One example is the study of twenty-seven patients with 33 zirconia fixed partial
dentures (FPDs) examined during five-years, reported by Sailer et al [6], where 15.2% of the
FPDs presented chipping of the veneering porcelain. Likewise, Vult von Steyern et al [7],
reported a study of 23 five-unit FPDs that were fabricated for 18 patients on a total of 56
abutments. They were clinically followed for 24 months, where 15% of the failures were
observed in the porcelain. Tinschert et al [15], reported the study of 46 patients with FPDs
monitored for 3 years. In four cases chipping in the veneering porcelain occurred.
Kelly [16] calculated the mean failure load versus days of function, assuming 1 million cycles
per year, from data collected of analogues of single-unit prostheses fabricated according to
common dental laboratory and clinical practices. Under wet cyclic loading a prediction from 1
to 7 years to failure was determined.
All the previous studies report failure after a time under service, but do not discuss in detail
the failure from a material viewpoint, where contact fatigue and stress corrosion cracking
(SCC) of the porcelain may be a relevant cause of failure. Stress corrosion cracking refers to
crack propagation of small pre-existing flaws under sustained loads usually in presence of
water [1,14] and contact fatigue refers to the crack extension due to the degradation of the
microstructure by cyclic contact loading [1,19,20], and it is usual that both mechanisms act
simultaneously.
During the mastication process, the magnitude of the forces range between 3 and 364 N over
cuspal radii of 2–4 mm with chewing cycles spanning from 0.25 to 0.70 seconds. It is
estimated that the contact loading period per each day is of 15 to 30 minutes [4,12,16-18].
Therefore, cyclic contact loads can be a relevant factor for damaging the dental prosthesis.
Masticatory loads and curvature of the teeth can be closely simulated by Hertzian contact
loading, where the concentrated stress field result in progressive local damage accumulation
[2, 11, 19-21]. This technique permits to follow the full evolution of contact damage, from
initial cracking to final failure [16, 22].
Recently, we have presented [19] the study of monotonic contact of the feldespathic porcelain
on dental zirconia, where the first ring crack appeared in a load range between 40 and 200 N
depending on the sphere radii and additional concentric ring cracks and small radial crack that
caused removal of material between cracks was observed with higher loads between 80 and
750 N.
The aim of the present work is to investigate the contact fatigue response of
feldspathic porcelain on dental zirconia, by employing static and cyclic Hertzian indentation
tests to assess the stress corrosion cracking and the fatigue of the material, respectively.
2. Experimental Procedure
2.1. Material Preparation
Tetragonal zirconia polycrystalline powder stabilized with 3 mol% yttria (3Y-TZP, Tosoh,
Japan) was isostatically pressed at 200 MPa to cylinders of 10 mm in diameter, which were
sintered for 1hour at 1200 oC. The cylinders were cut in discs of 3 mm thick and polished
with a 120 grit SiC disc and fired for 5 min at 700 oC (Vita Vacumat 40T furnace) for
cleaning.
The discs were immersed for 2 min within YZ Coloring Liquid LL1 (VITA Zahnfabrik,
Germany) and sintered for 2 hours at 1450 oC. Final grain size was 0.3  0.1 m (measured
by linear interception method) with a density of 6.03  0.01 g·cm-3 (measured by Archimedes
method), yielding 99.9% of the theoretical density.
Subsequently, zirconia discs were coated with feldspathic porcelain (Vita VM9, Zahnfabrik
VITA, Bad Sackingen, Germany). A porcelain coating of around 150 m porcelain was
applied and fired for one minute at 950 °C. Then, a thicker layer of around 500 m was
applied and fired at 910 oC for one minute. Both treatments were carried out in a Vita
Vacumat 40T furnace. Samples were finally lightly polished with 30 m diamond suspension
followed by colloidal silica.
2.2. Mechanical Testing
The Hertzian contact loading tests were performed using a servo-hydraulic Instron universal
machine (model 8511) with tungsten carbide-cobalt spheres (WC-Co) with a ratio (Re) of 1.25
mm.
Three different types of tests were performed: 1) Static tests on air, 2) Cyclic tests on air and
3) Cyclic tests on artificial saliva medium with a composition given in table 1 and a ph of 6.7
[23].
<<table 1>>
The monotonic critical load to obtain the first brittle damage (full ring crack), Prc=70 N have
been previously reported [19]. Therefore, static tests were performed applying a maximum
loads, Pmax lower than Prc in air (relative humidity ∼ 40%) and at a load rate of 5 Ns-1. After
removing the load, the indentation imprint was observed by optical and laser confocal
microscope (Olympus BX41M and Olympus LEXT OLS 3100 respectively). If no ring
cracking was observed, a new test was performed in another location with longer holding
time, for a maximum time of 104 seconds. Once damage was detected, other tests were done
with shorter contact time (less than 5%) in order to determine the time for full ring cracking.
The time for ring cracking, trc was defined as that for which at least three tests presented this
damage.
The ring cracking under artificial saliva was evaluated by applying the same loads, Pmax, that
in static test.
Cyclic loading was applied by means of a sinusoidal waveform at a frequency, f= 10 Hz and
with a load ratio (R = Pmin/Pmax) of 0.1. The number of cycles for ring cracking (Nrc) were
determined by the same methodology as before.
The samples for cyclic contact in artificial saliva were tested after several days of immersion
in same medium at 37 oC. Fig.1 presents an image of the set-up for the artificial saliva test.
<<Fig. 1>>
3. Results
3.1. Stress Corrosion Cracking Under Static Contact Loading in Air
Fig. 2, presents the apparition of damage as a function of time under static contact tests in air
with constant applied loads Pmax, lower than the critical load Prc=70 N. From this figure it is
seen that the material is sensitive to contact SCC even in presence of air, as full-ring crack
appears for Pmax values less than Prc. Fig. 3, shows the images of the full-ring crack and
partial ring crack of the points indicated in Fig. 2. This may be a consequence of the relative
humidity present in the environment.
The crack growth as a function of time (dc/dt) under static loading is related to the stress
intensity factor KI, through:
𝑑𝑐
𝑑𝑡
𝐾
𝑛
= 𝜐0 (𝐾 𝐼 ) , 𝐾𝐼 < 𝐾𝐼𝑐
𝐼𝑐
(1)
Where o is a material dependent constant, KIc is the fracture toughness of the material and n
is the crack velocity exponent [3, 20-22, 24-26].
In the case of contact loading, the exact value of KI is not analytical due to the high variations
of the stress field. A relationship between the critical load and the cracking time for prolonged
times has been proposed by Pavon et al assuming that the ring crack coalesces from the
different pre-existing microcracks [25].
𝑃2𝑛⁄3 𝑡𝑟𝑐 = 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡
(2)
Calculating the slope of the curve in Fig. 2, a value n =18 was obtained. This value was
contained in the values range 9 to 60 of stress corrosion cracking exponent for ceramics
reported previously in literature [20, 25]
<<Fig. 2>>
<<Fig. 3>>
3.2. Cyclic Hertzian loading in air and in artificial saliva
Cyclic tests in air and artificial saliva environments were performed to determine the number
of cycles for ring cracking (Nrc), as presented in Fig. 4. The results of these tests show that the
dental porcelain is sensitive to cyclic loading and that the damage induced in presence of
artificial saliva is greater than in air. It is important to recall that when a cyclic test is
performed, both SCC and fatigue are operating at the same time. When performing the tests in
artificial saliva, it is expected that SCC will be more pronounced due to the humid
environment.
<<Fig. 4>>
To observe the damage evolution in air and in artificial saliva environments, a cyclic load
with a maximum constant load of 50 N was applied and the damage induced was followed
with the number of cycles. The results are presented in Fig. 5. In both environments the first
damage observed was a ring crack, although in saliva the ring crack appeared sooner. As the
number of cycles progresses, several other damages become evident, specially the generation
of secondary concentric ring cracks which may develop into cone cracks, probably produced
by hydraulic pumping [27,28], and wear tracks produced by the elastic mismatch between the
material and the indenter. A few radial cracks may be also observed. As the number of cycles
is increased, multi-ring cracking and some material removal between ring cracks can be
appreciated. It has to be noted that the damage typology and sequence will be influenced by
the substrate. Being zirconia a relative stiff and tough material, damage will be produced
manly at the coating. In case that a more compliant substrate is used, other type of damage
can appear, such as delamination or radial cracking at the interface.
The relationship between the critical load and the number of cycles for cracking (Nf) cannot
be represented by a single potential law. It is clear that the experimental points at high
maximum load follow a different slope than at small loads. For each part of the curve a
straight line can be plotted of the type [29]:
m
Pmax
·N f  constant
(3)
With an exponent m which is similar to that for the high static loads studied, while for small
cyclic loads the value of m is equal to calculating the slope of the curve in Fig. 4. The m
exponent value was found equal to 39 [29, 30].
In any case, no large radial cracks and no inelastic deformation were observed, measured by
confocal microscopy.
<<Fig. 5>>
4. Discussion
From the results presented here on static and cyclic contact tests, the first question that may
arise is whenever there is really a true fatigue effect produced by cyclic loading, or the results
of cyclic contact testing may be attributed to the stress corrosion cracking of the glassy phase.
The effect of the SCC can be deconvoluted from the fatigue effect following the work of
Evans and Fuller [20], by assuming that the failure under cyclic loading is due to the same
population of defects and the crack growth mechanism present in the static load, the
loading/unloading in the Hertzian contact test simulate a series of small increments of
constant load, and that the compressive loads do not induce crack growth.
Under these assumptions, if the only mechanism was SCC without any fatigue, the time to
fracture under cyclic loads can be calculated to be around 10 times lower than under static
loading at the same maximum load. This prediction is presented in Fig. 6, together with the
experimental curves for static and cyclic loads.
In this figure, it can be seen that the experimental curve obtained under cyclic loading is
below the theoretical curve which is calculated under the assumption that only SCC is the
degradation mechanism. This clearly indicates that there is a mechanical fatigue effect, So,
despite the fact that SCC is the leading mechanism on degradation, mechanical fatigue is also
a contributing factor to the growth of cracks.
<<Fig. 6>>
A comparison between the morphology of a ring crack in air and saliva medium is presented
in Fig. 7, showing the images of ring cracks in air under static contact for 2.6 min and
artificial saliva under cyclic contact for 6000 cycles for the same maximum constant load
(P=60 N) for both situations. In this figure it can be seen that both cracks are similar, but the
one produced by cyclic loading presents a higher amount of tortuosity, probably due to the
interaction of the leucite particles with the crack. The cyclic tests under artificial saliva
produced an earlier apparition of cracking in the samples. In figure 7, it is seen that the crack
path is slightly more tortuous, due to the relative motion between crack faces and the possible
interaction between leucite particles and the crack tip.
<<Fig. 7>>
During cyclic loading, inner ring cracks appear after a high number of cycles. These inner
cracks are exclusive of cyclic loading and will develop into cone cracks inside the material.
This fracture is due to hydraulic pumping of fluid inside the material due to the fluctuation of
stress fields from compressive to tensile as the load is cycled, as described by Lawn et al [27,
28] for other brittle materials.
From the experimental results, one could attempt to estimate the lifetime of these materials
under more realistic conditions. First of all, although for low times, stress corrosion cracking
is the most relevant crack forming mechanism, for longer times the cyclic degradation of the
material is the relevant factor in producing fracture. In addition, as said before, under cyclic
conditions some secondary cracks can appear which will also be detrimental for the structural
integrity of the component. Therefore, for estimating the lifetime of these materials the curve
obtained for cyclic loading under artificial saliva has been chosen. These results are in
agreement with the ones found by Kim et al. [4] using sliding contact loading on bilayers and
trilayers systems of porcelain/zirconia and polycarbonate.
One further consideration to take into account is the fact that the testing has been performed
with a WC-Co hard metal sphere, whereas under real conditions the contact counterpart will
be food, teeth or other dental prosthesis which have a lower elastic modulus. This difference
in elastic modulus will provoke a difference in the critical load necessary for apparition of
cracks. If the friction between indenter and material is assumed to be low due to the presence
of a fluid, ring crack fracture will be produced by a the critical tensile stress (tm) along the
contact perimeter:
𝜎𝑡𝑚 =
1
2
(1 − 2𝜈) p
(3)
0
This stress is proportional to the mean pressure between the indenter and the material (p0):
p =
0
1 4 2⁄3
𝑃
𝜋𝑎2
= 𝜋 (3)
𝐸 eff
(
𝑅
2⁄3
)
𝑃1⁄3
(4)
where a is the contact radius, and Eeff is the effective modulus between the sample and the
indenter which it is defined as:
1
𝐸 eff
=
1−𝜈 2
𝐸
+
1−𝜈𝑖2
𝐸𝑖
(5)
If the contact loading is produced between two dental prosthesis the contact pressure for
chewing loads of 40-250 N [31] is 797-1469 MPa, which is significantly lower than the
contact pressure of an WC-Co indenter (1166-2148 MPa for the same loads), if the contact is
produced between a natural teeth and a prosthesis, the range of contact loads is between 3-364
N and the contact pressure ranges between 367 and 1819 MPa.
In Fig. 8 the contact pressure against days to failure is presented, assuming that the number of
chewing cycles per day is up to approximately1400 [33], including the experimental points
and the obtained fitting. Using the contact pressures above estimated, the lifetime for a
prosthesis which is in contact against teeth or dental prosthesis is around 1 to 4 years while in
the case of contact against a softer material is much larger than the lifetime of a person since
the pressures range is around 30 and 151 MPa, for an average of 2·106 to 4·106 years, clearly
an infinite life criteria for the material. The calculated failure time of between 1 and 4 years is
agreement with clinical reports by different authors [6, 7, 12, 14, 33-35].
<< Fig. 8>>
Therefore, one of the causes of failure of dental porcelain coatings on zirconia prosthesis can
be attributed to the cyclic contact loading that the material suffers against other teeth or dental
prosthesis, which is the case of bruxism. In absence of significant bruxism, cyclic contact
loading against food will not produce significant damage on the material, and can be
discarded as a source of failure. In case of bruxism, it is then recommended to protect the
dental prosthesis from suffering cyclic contacts (e.g. by an occlusal splint).
5. Conclusions
Feldspathic porcelain veneer is sensitive to both static contact loads and cyclic loads. Both
mechanisms act simultaneously and are more relevant under artificial saliva environments.
Only ring crack and cone crack were observed in the material, without inelastic deformation
or other secondary cracks.
Extrapolation of the results show that cyclic contact loading of the porcelain against teeth or
other ceramic prosthesis can be the cause of premature cracking and failure. Therefore, proper
guard against hard contact can be a mean of increasing the lifetime of the coating.
Acknowledgments
The authors acknowledge the grant received through of MAT2008-03398/MAT project of
Ministry of Science and Innovation of Spain. A. R would like to thank the grant received by
the Generalitat de Catalunya (2009SGR01285).
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