J Dent Res 74(9): 1591-1596, September, 1995
Shear vs. Tensile Bond Strength
of Resin Composite Bonded to Ceramic
A. Della Bona* and R. van Noort
Department of Restorative Dentistry, University of Sheffield, S10 2TA, Sheffield, United Kingdom; *to whom correspondence and reprint
requests should be addressed, at The University of Passo Fundo, School of Clinical Dentistry, Department of Restorative Dentistry, 817
Teixeira Soares St., 99010-080, Passo Fundo, RS, Brazil
Abstract. Since the mode of failure of resin composites
bonded to ceramics has frequently been reported to be
cohesive fracture of either ceramic or resin composite rather
than separation at the adhesive interface, this study was
designed to question the validity of shear bond strength
tests. The reasons for such a failure mode are identified and
an alternative tensile bond strength test evaluated. Three
configurations (A, conventional; B, reversed; and C, all
composite) of the cylinder-on-disc design were produced
for shear bond strength testing. Two-dimensional finite
element stress analysis (FEA) was carried out to determine
qualitatively the stress distribution for the three
configurations. A tensile bond strength test was designed
and used to evaluate two ceramic repair systems, one using
hydrofluoric acid (HF) and the other acidulated phosphate
fluoride (APF). Results from the shear bond strength tests
and FEA showed that this particular test has as its inherent
feature the measurement of the strength of the base material
rather than the strength of the adhesive interface. In the
tensile test, failure invariably occurred in the adhesive layer,
with HF and APF showing a similar ability to improve the
bond of resin composite to ceramic. It is concluded that the
tensile bond strength test is more appropriate for evaluating
the adhesive capabilities of resin composites to ceramics.
Key words: tensile, shear, bond strength, finite element.
Received August 25, 1994; Accepted July 5, 1995
The dental profession still has no universally accepted bond
strength test for resin composites bonded to ceramic, despite
the great amount of research papers on this topic over the
last decade (Stangel et al., 1987; Lacy et al., 1988; Nicholls,
1988; Tjan and Nemetz, 1988; Diaz-Arnold et al., 1989; Lu et
al., 1992). Tensile, flexural, and shear tests (Nicholls, 1988;
Bailey, 1989; Della Bona and Northeast, 1994) have been
used to measure the resin-ceramic bond strength, with the
shear bond strength test being the most popular.
A notable feature of some recent publications (Diaz-
Arnold et al., 1989; Sorensen et al., 1991; Della Bona and
Northeast, 1994) is the observation that the failure mode is
often cohesive within the ceramic base rather than at the
adhesive interface, on the basis of which it has been
suggested that the bond strength exceeds the cohesive
strength of the ceramic. This ignores the nature of the stresses
generated and their distribution within the adherence zone,
which can have a profound influence on the mode of failure.
Finite element stress analysis (FEA) has been used to study
the sensitivity of bond strengths to specimen design and
changes in testing conditions (Anusavice et al., 1980; van
Noort et al., 1989; van Noort et al., 1991; Shiau et al., 1993).
These studies show that there is a need for a more critical
approach to the design of appropriate tests for evaluating the
bond strength of resin composite to ceramic if the desire for a
standardized test procedure is to be achieved. For this
objective to be accomplished, a careful examination of bond
strength tests is mandatory for correct interpretation of the
bond strength data.
The contention of this study is that the shear bond strength
test is inappropriate and inadequate for the in vitro
assessment of resin composite bonded to ceramic,since failure
occurs in the ceramic base and not at the adhesive interface. It
is hypothesized that this mode of failure is due to the
generation of high tensile stresses within the samplebase as a
consequence of the non-uniform stress distribution generated
in the shear test arrangement. The aims of this study were to
Della Bona & van Noort
examine a variety of shear bond strength test arrangements
and to assess, by FEA, the effect of the stress distribution on
the shear bond strength and failure mode. A tensile bond
strength measurement technique for ceramic bonded to resin
composite is proposed as a more suitable alternative.
Materials and methods
The materials used consisted of a feldspathic ceramic (Vita
VMK68, Vita Zahnfabrik, Bad Sackingen, Germany), an
adhesive and a hybrid resin composite (Prisma Universal Bond-
3 and Prisma APH, Dentsply Limited, DeTrey Division,
Weybridge, Surrey, UK), solutions of 9.6% hydrofluoric acid gel
(HF) and 4% acidulated phosphate fluoride (APF), plus a silane
coupling agent (Mirage Dental Systems, Chameleon Dental
Products Inc., Kansas City, KS, USA).
Shear bond strength sample preparation
Three configurations of the same specimen design were
produced with identical adhesive interfaces in terms of both
geometry and surface area. The first followed the conventional
configuration (Group A), consisting of a ceramic base to which a
resin composite cylinder was bonded. In the second
configuration (Group B), the materials were reversed such that
the specimen base was made of resin composite bonded to a
ceramic cylinder. The third Group (C) was made solely out of
resin composite, thus being devoid of any adhesive interface.
To produce the conventional configuration (Group A), we
made ten ceramic discs 10 mm in diameter and 3 mm in height.
Vita VMK68 A2 dentin shade powder (batch #1485) was mixed
with the Vita Modeling Liquid (batch #56331) and condensed into
the mold by means of a brush and vibration (Aranda and Barghi,
1988; Evans et al., 1990). The green bodies were sintered according
to the manufacturer's recommendations in a Vita Vacumat 200
oven (number 93468, Vita Zahnfabrik, H. Rauter GmbH & Co.,
Germany). The ceramic discs were embedded in epoxy resin
(Metset type FT, Buehler, Coventry, UK), allowed to set for 24
hours, and ground flat by means of 400- and 600-SiC paper. All
specimens were ultrasonically cleaned in distilled water for 5 min
and dried. The ceramic surface was etched with HF for 2 min,
washed thoroughly for 2 min under running tap water, and air-
dried. Silane bond enhancer was applied by a clean brush and
allowed to dry fully. The specimen was then placed in a holding
jig, where the resin composite cylinder was built up through a
hole (R = 3 mm, h = 4 mm) located centrally in a disk-shaped
silicone rubber mold. A thin layer of Prisma Universal Bond 3 was
applied with a brush and light-cured with a Visilux 2 light-curing
unit (Dental Products Division/3M, St. Paul, MN, USA) for 10 sec.
Prisma AP.H (universal shade) was applied in two increments,
with each one cured for 20 sec. The silicone rubber mold was
removed and the sample cured for an extra 40 sec from different
For the reversed configuration (Group B), two silicone rubber
molds were made from machined acrylic resin models (Fig. 1). Ten
Vita VMK68 A2 dentin ceramic rods (R = 3 mm) were made
following the manufacturer's instructions. All rods were reduced
to a height of 4 mm, had one surface flattened by 400- and 600-SiC
paper on a special holding device, were ultrasonically cleaned in
distilled water, and were dried. HF and silane coupling agent were
applied to the flattened ceramic surface following the same
procedures as described above. The ceramic rod was then placed
Acrylic resin model 1
all composite specimen
Silicone rubber mould 1
Silicone rubber mould 2
Acrylic resin model 2
Figure 1. Acrylic resin models (1 and 2) and their respective silicone
rubber molds. Mold 1 was used to build the reversed and all-
composite specimens. All specimens were embedded in epoxy resin
in mold 2.
ready for shear bond
in silicone rubber mold 1, and a thin layer of adhesive was applied
to the prepared ceramic surface by brush. This layer was light-
cured for 10 sec, and Prisma AP.H was packed into the mold in
two 1.5-mm-thick increments, cured for 20) sec each. The sample
was carefully removed from mold 1, and an additional 40 seconds'
curing was applied to the resin composite base from different
directions. The sample was then placed into mold 2 (Fig. 1), epoxy
resin was poured into it, and it was allowed to set for 24 hours.
The all-composite samples (Group C) were made solely out of
Prisma AP.H which was placed incrementally down the central
hole of mold 1 (Fig. 1) and gradually built up to create a cylinder-
on-disc specimen. This was carefully removed from the mold, and
40 seconds' additional curing time was applied from different
lateral directions. The specimen was then placed in mold 2 (Fig. 1),
embedded in epoxy resin, and allowed to set for 24 hours.
Samples from all groups were stored in distilled water at
37°C for 3 days before shear bond strength testing occurred.
Shear bond strength test
A Lloyd M5K universal testing machine(I.J. Lloyd Instruments
Ltd., Warsash, UK), with the knife edge placed as close as possible
to the junction between the base and the cylinder, was used for
testing. A cross-head speed of 0.5 mm/mmi
maximum load recorded for each specimen. The nominal shear
bond strength was calculated by P/A, where P is the load at failure
and A is the cross-sectional area of the cylinder. The fracture
surface of each specimen was examined under x40 magnification
so that the mode of failure could be determined. The data were
analyzed by one-way analysis of variance (ANOVA).
was used and the
Finite element stress analysis (FEA)
The stress distributions for the three shear test configurations
were determined from a two-dimensional plane-strain computer
model of a central section with dimensions identical to those of
the experimental samples (FINEL, Babcock Plower Ltd., London).
The nodes at the sides and bottom of the base material were
constrained in both the x and y directions. The Ploisson's ratio (pi)
and elastic modulus (E) for the ceramic and resin composite
were p = 0.30, E = 83 GPa and p = 0.25, E = 8 GPa, respectively
IDeiit Rcs 74(9) 1995
Resin-to-Ceramic Bond Strength Testinig
Figure 2. A couple of grooved metal rods and a set of stainless steel
clamps containing a three-point gripping system at one end and a
holding mechanism for the testing machine at the other end. The
three round-tip screws were designed to fit into the specimen
groove and tightly grip it during tensile bond strength testing.
(Sakaguchi et al., 1992). A shear load of 10 N was applied parallel
to the base at a position 0.2 mm above the surface of the base.
Tensile bond strength test design and sample preparation
The tensile bond strength specimen design consists of two rod
specimens of uniform cross-section, bonded together on their
ceramic surfaces and pulled apart in the universal tester. A special
set of clamps was fabricated in stainless steel containing a three-
point gripping system for the specimens at one end and a holding
mechanism for the testing machine at the other end (Fig. 2).
Forty Ni/Cr rods (R = 4 mm) were made with Talladium-V
alloy (Talladium Inc., Valencia, CA, USA). A groove was cut
around each metal rod at a distance of 2 mm from one of its
ends to accommodate the three round-tip screws of the clamps.
The non-grooved end of the metal rod was grit-blasted with 50
pm aluminum oxide and ultrasonically cleaned in distilled
water. After metal degassing, Vita VMK68 opaque (batch #1222)
and dentin ceramic (batch #1485) were applied and fired
according to the manufacturer's instructions. By 600-grit SiC
paper used on a special grinding device, the ceramic surface
was flattened, resulting in a ceramic extension to the metal rod
measuring 2 mm in height and 3.45 mm in diameter. Specimens
were ultrasonically cleaned in distilled water, divided into two
groups of 20 at random, and treated as follows:
Specimens in group I had their ceramic surfaces treated with
9.6k/ HF for 2 min, washed thoroughly under running tap water,
and dried with oil-free air. Silane coupling agent was applied and
allowed to evaporate. A thin layer of Prisma Universal Bond 3
was brushed onto a pair of treated specimens and light-cured for
10 sec. Prisma AP.H was applied to the surfaces, the screw of the
alignment jig was tightened to produce a thin resin composite
layer (Fig. 3) and light-cured for a total of 120 sec from different
directions (Blackman et al., 1990). No attempt was made to
control the thickness of the adhesive layer other than by the
amount of pressure being applied by the screw. We relied
essentially on the film-thickness characteristics of the resin
composite, and since the same procedure was used throughout,
we assumed that this would not have varied greatly. The excess
resin material was cut down to the ceramic level by resin
composite polishing burs. Samples were stored in distilled water
at 37°C for 3 days before undergoing tensile bond strength testing
Figure 3. Twin specimens were placed in the alignment jig, and the
adhesive complex layer was light-cured from different directions
in a Lloyd M5K tester at a cross-head speed of I mm/mim.
Specimens in group 2 were treated following the same
procedures as used above, except for the application of 4% AI'F
instead of HF for 2 min. Since just two groups of data were
present, the independent Student's t test was selected for the
Selected specimens were further examined under the scanning
electron microscope (Philips SEM 501, Eindhoven, The Netherlands).
The values for the nominal shear bond strengths of the three
specimen configuration groups are presented in Table 1. The
nominal shear bond strength for Group A was significantly
different from those of Groups B and C (P < 0.001). The
results for Group B and Group C were not significantly
different at P < 0.001.
Cohesive fractures of the ceramic base and interfacial
adhesive failures occurred in equal proportions in Group A.
In group B, 80% of failures were cohesive fractures of the
resin composite base, and for group C, all failures were
cohesive fractures of the resin composite base. The cohesive
fracture path is shown schematically in Fig. 4, with the
fracture running in an arc from the surface of the base at a
point to the left of the point of load application and under
the adhesive interface.
The contours of the vertical((a,v)and horizonital ((ar)
stresses resulting from an applied shear load are shown in
Figs. 5 and 6, respectively, for the FEA model of the
conventional configuration. As expected, the pattern of stress
in this shear bond strength test specimen design is highly
non-uniform. A notable feature is that the maximum tensile
stress in the vertical direction (ar
interface nearest the point of loaY application, as reported
previously (van Noort et al., 1989), which is the result of a
bending moment as suggested by Shiau ct al. (1993). Yet the
highest tensile stress was found in the horizonital((r5,,)
direction and not in the adhesive layer or the cylinder but at
the surface of the base just to the left of the cylinder (Table 2).
The stress distributions for the other two configurations were
found to be very similar in terms of where the maximum
tensile stresses occurred, although there were slight
differences in the stress patterns. Also, the relative values for
the maximum tensile stresses were different (Table 2), due to
) occurs at the adhesive
jDctit Rcs 74(9) 1995
Della Bona & van Noort
Table 1. Shear bond strength data (MPa)
A - Conventional
B - Reversed
C - All-composite
The vertical line denotes no significant difference between Groups,
as determined by ANOVA (P < 0.001), with a post hoc Tukey's
honestly significant difference (HSD) test.
8.25 - 13.64
15.06 - 23.07
15.36 - 24.16
the different elastic moduli assigned to the base and the
cylinder in the three configurations.
The nominal tensile bond strengths for the HF- and APF-
etched experimental groups are presented in Table 3. No
significant difference was found between the nominal tensile
bond strengths of Group 1 and Group 2 (P < 0.05). All
specimens in both Groups fractured within the adhesive
interface complex. The fracture was always at or near the
adhesive/ceramic interface and never in the bulk of the resin
composite or the ceramic. Surface flaws exist in all materials,
whereas interfacial flaws can arise only when two materials
are stuck together. From the example in Fig. 7, areas of the
etched ceramic surface can be identified by their scalloped
appearance, together with regions of resin fracture where
resin tags have formed in the etched ceramic surface. This
suggests that failure is governed by interfacial as opposed to
surface or bulk flaws in the ceramic or the resin composite.
The adhesive layer thickness may have an influence on the
measurement of bond strength if failure occurs from surface
flaws or flaws within the bulk of the resin-composite. This is
a factor which may need further exploration. There were no
cohesive fractures of the ceramic, yet this was the dominant
mode of failure for the shear bond strength test.
The shear bond strength test arrangement has been the most
Figure 4. A schematic representation of the mode of failure for the
shear bond strength arrangement due to high horizontal tensile
stresses at the surface of the base (thin arrow), exceeding the tensile
vertical stresses at the adhesive interface.
common laboratory technique for evaluating adhesives for
resin-bonded ceramic restorations and ceramic repair
systems. It has been shown that shear bond strength
measurements are very sensitive to the method of application
of the adhesive and design of the testing arrangement
(Anusavice et al., 1980; van Noort et al., 1989; van Noort et al.,
1991). These factors can lead to false interpretation of the
resultant bond strength data. This study shows that
significant differences in shear bond strengths are obtained
for different sample configurations when an identical
geometric design and an identical adhesive interface are used.
Use of a ceramic base resulted in a shear bond strength which
was nearly half that obtained with a resin composite base,
even though the bonded areas were identical. Itgives some
credence to the expression: "Imagine a bond strength value,
and a test arrangement can be designed to produce such a
result." The fact that a virtually identical shear bond strength
was obtained for the all-composite sample compared with the
Figure 5. FEA close-up contour plot of the vertical stress distribution
(ayo)for the conventional shear test arrangement (configuration A).
The black arrow shows the point of load application. The numbers
correspond to the stress values (MPa) in the legend.
Figure 6. FEA close-up contour plot for the horizontal stress
distribution(uxx) for the conventional shear test arrangement
(configuration A). The black arrow shows the point of load
application. The numbers correspond to the stress values (MPa) in
Start of fracture
jDent Res 74(9) 1995
Resin-to-Ceramic Bond Strength Testing
Table 2. Maximum tensile stresses for each computer model (MPa)
Stress DirectionMaximum Value
composite base with a ceramic cylinder shows that the test
does not actually assess the quality of the adhesive bond. This
is consistent with observations of the modes of failure, which
were predominantly cohesive fractures of the base material.
Notwithstanding the lack of sophistication of the FEA
models used, which could be much improved by mesh
refinement and development of a 3-D model, the qualitative
results obtained clearly indicate that all three configurations
developed tensile surface stresses within the base very close
to the cylinder-base interface edge nearest to the applied
shear load. Since these surface tensile stresses ((u>x) were
considerably higher than the interfacial vertical tensile
probable that fracture was initiated from the surface of the
base, as confirmed by the many cohesive fractures observed
and as suggested by Anusavice et al. (1980). The shear bond
strengths determined in this study are therefore governed
by the resistance of the base to surface tensile stresses, i.e.,
the base material's tensile strength, rather than the strength
of the adhesive interface. This will be the case for as long as
the adhesive interface is sufficiently strong to resist its local
stressing conditions. Apparent differences in bond strength
values when the same ceramic base is used can arise simply
because of differences in its surface finish. When a ceramic
base is grit-blasted, it will have a different resistance to
) by approximately a factor of three, it is highly
Figure 7. A scanning electron microscope view of the fracture
surface of a tensile bond strength specimen from the HF-etched
group. The scalloped appearance is typical of an etched ceramic
surface (b), and areas of resin fracture (a) are also readily
identifiable where resin tags have formed in the etched ceramic
surface. Bar = 5 microns. The original magnification is 5000.
Table 3. Tensile bond strength data (MPa)
I - 9.6% HF etchant
2 -4% APF etchant
The groups do not show a difference at the 0.05 significance level,
as determined by independent Student's t test.
fracture from the surface tensile stresses than if it were
etched or polished. Another contributory factor will be the
moduli causing changes in the stress distribution, such that
if a base or cylinder with a different elastic modulus is used,
one could get yet another bond strength value, which again
may have nothing to do with the adhesive interface. Thus, it
is reasonable to suggest that the surface tensile stresses of
the base material offer the best possible explanation for the
shallow cohesive fractures of the ceramic base as observed
in these and other shear bond strength tests.
Though cohesive failures starting near the load application
point were most frequent, some failures at the adhesive
interface did occur in Groups A and B. This is related to the
statistical nature of the strength characteristics of the materials
used, all of which are highly brittle. Surface flaws, internal
material flaws-in either the base material, the adhesive layer,
or the cylinder-and large interface flaws can all play an
important role in determining failure sites. Sometimes failure
will occur at sites of relatively low local stress merely because
there is a particularly large flaw so oriented in a stress field as
to be ideal to cause fracture. Possible sites from which failure
may start are therefore highly unpredictable, since this will
depend on flaw size and distribution in relation to the stress
distribution. Thus, statistically, one would expect some
fractures to occur at the adhesive interface if there happens to
be a flaw of a substantial size and ideal orientation to
propagate. It is noteworthy that for the all-composite
arrangement (Group C), none of the failures occurred in the
interface region, where of course there would be no interfacial
flaws, since there was no adhesive.
These findings easily justify the search for a more
representative bond strength test with a simpler geometric
design and consequently a more uniform stress distribution.
Nicholls (1988) described a tensile bond strength
arrangement similar to the one designed for this study. In
the present study, all failures for the tensile bond strength
test occurred within the adhesive interface complex,
regardless of the bonding procedure. This means that the
data obtained provide a more representative measurement
of the tensile bond strength of the bonding area rather than
the strength of a particular base material, as probably
happens during the shear bond strength test. Although
stress inhomogeneities due to geometry are avoided, the
interfacial stress should not be assumed to be uniformly
tensile. FEA work has shown that some non-uniformity
remains due to the changes in elastic moduli (Aivazzadeh et
al., 1988). While these are considerably less than those
occurring in the shear test arrangement, it is by no means
suggested that the tensile bond strength test is ideal; other
tests should be considered equally for their suitability (Tam
Txxdue to differences in the elastic
jDcnt Res 74(9) 1995
Della Bona & van Noort
and Pilliar, 1993: Degrange et al., 1994).
Etching techniques have been widely used to improve the
bond strength of ceramic to resin composite (Calamia et al.,
1985; Sorensen et al., 1991; Della Bona et al., 1993). HF and
APF are the most commonly used etchants for this purpose.
Although no significant difference (P < 0.05) was found
between the tensile bond strengths for specimens etched with
9.6% HF and those of specimens etched with 4% APF, the
data in the group with 4% APF showed a wider spread than
in the one etched with 9.6% HF. This suggests that HF etching
may well produce a more reliable and consistent result.
However, this could not be confirmed, since the sample size
was too small for a Weibull analysis. Comparison of these
results with those of other studies is difficult for several
reasons. Clearly, a comparison with shear bond strength data
would be invalid, since one would not be comparing like with
like. The lack of a standardized tensile test procedure, plus
the non-existence of previous reports on tensile bond
strengths where 4% APF etchant was used, also makes such a
task impossible. Even when a very similar test arrangement
was used (Nicholls, 1988), the materials and research protocol
were different. Nevertheless, ceramic repair systems provide
some ability to bond to ceramic, although the mode of failure
for the tensile bond strength tests indicates that the adhesive
interface still represents the weak link in the system. Until
such time that the tensile bond strength required for a
clinically acceptable ceramic repair has been determined, its
measurement can be used as only a crude ranking parameter,
such that, for the present, long-term clinical performance of
the ceramic repair materials remains the ultimate test.
Within the limitations of the present in vitro study, it can be
conduded that shearbond strength data in the cylinder-on-disc
experimental design are governed by the cohesive strength of
the base material used and not by the bond strength of the
adhesive interface. Also, the cohesive failure of the base of the
specimen is an inherent feature of the geometry of the shear
bond strength test arrangement. Therefore, the shear bond
strength procedure is inadequate as a means of assessing the
quality of the adhesive bond of resin composite to ceramic, and
perhaps the time has come for it to be abandoned in favor of a
test which more genuinely measures the quality of the
adhesive interface. Failure within the adhesive interface
complex for the tensile bond strength test is evidence that this
type of test arrangement is more appropriate for evaluating the
bond strength of resin composite to ceramic.
This work was supported in part by the Department of
Restorative Dentistry at the University of Sheffield and is
based on Dr. Della Bona's thesis, which was submitted to
the graduate faculty, The University of Sheffield School of
Clinical Dentistry, in partial fulfillment of the requirements
for the MMedSci degree.
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