Open Journal of Stomatology, 2013, 3, 307-313 OJST Published Online September 2013 (
Diminished fracture initiation sites in ceramic layers
bonded to glow-discharge treated substructure
Hyeongil Kim1*, Edward A. Monaco Jr.1, Frederick McIntyre2, Elaine L. Davis3, Robert E. Baier3
1Department of Restorative Dentistry, University at Buffalo, New York, USA
2VA Western New York Healthcare System at Buffalo, New York, USA
3Department of Oral Diagnostic Sciences, University at Buffalo, New York, USA
Email: *
Received 10 June 2013; revised 10 July 2013; accepted 31 July 2013
Copyright © 2013 Hyeongil Kim et al. This is an open access article distributed under the Creative Commons Attribution License,
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
A mechanically retentive structure and meticulous
surface cleanliness are critical factors in providing
fracture resistance and clinical success of metal ce-
ramic restorations. This investigation compared the
porcelain/metal interfaces of deliberate compressive
fractures of ceramic crowns between conventional
preparation and application of the Radio Frequency
Glow Discharge Treatment (RFGDT) before each
bonding step. It evaluated RFGDT’s capacity to im-
prove wetting effectiveness and minimize porosity.
Twelve metal ceramic crowns were fabricated identi-
cally. RFGDT was applied to the metal substructures
of half the specimens before the ceramic layering
process. All specimens were fractured in the same
manner by an applied compressive force to simulate
dental occlusive failure. Fracture surfaces were in-
spected by light and scanning electron microscopy.
Quantitative analyses of images were performed to
identify numbers, locations of cracks, porosity pat-
terns, and other morphological correlates of the
fracture zones. There were significantly fewer voids
per millimeter at the interfaces in the RFGDT group
than in the non-RFGDT group (t = 2.377, df = 9, p =
0.021). There was a significant difference in the num-
ber of horizontal cracks per millimeter between the
groups (t = 2.132, df = 7, p = 0.035), with more cracks
occurring in the non-RFGDT group. RFGDT can
improve the integrity of metal ceramic crowns by
increasing the substratum surface energy, improving
porcelain wetting and spreading and thereby dimin-
ishing the numbers of interfacial voids available for
initiation of fracture. Routine application of RFGDT
should result in fewer cracks along metal/ceramic
interfaces in all restorative preparations.
Keywords: Porcelain Fracture; Glow Discharge
Treatment; Metal Ceramic Crown; Porosity;
Surface Energy
Ceramics are well known for their natural appearance,
durable chemical and optical properties, and as dental
restorative materials are frequent replacements for natu-
ral teeth because of their good wear resistance, chemical
inertness, surface texture and esthetics. A significant
early development in the use of porcelain for dental res-
torations was the fusion of porcelain to gold alloys to
overcome inherent brittle failure of ceramic crowns [1,2].
The fusion of porcelain to gold alloys allowed the es-
thetics of porcelain to be combined with the ductility,
strength, and toughness of the gold alloy. Restorations
fabricated with gold-dominated alloys and porcelain,
although clinically reliable, are being replaced with less
expensive metal alloy systems that also claim superior
physical properties in terms of strength and sag resis-
tance [3,4]. In recent years, dental porcelain and alloys
for bonding have undergone reformulation, resulting in a
greater variety of dental porcelains and ceramic casting
alloys, as well as a wider range of compositions and
costs. A remaining concern within the art and science of
casting, metal work, and porcelain bonding mechanisms
is the susceptibility of these systems to brittle fracture at
the ceramic/metal interface. In addition to structural
soundness, biologic and esthetic expectations of the pa-
tient require the resolution of issues concerning the dis-
coloration of silver-containing metal alloys, metal distor-
tion after coldwork [5], marginal discrepancies after por-
celain veneering [6-8], and the biological safety of nickel
[9] and beryllium-containing base metal alloys [10].
Alternative restorative systems have been developed
to overcome limitations of the conventional metal ce-
*Corresponding author.
H. Kim et al. / Open Journal of Stomatology 3 (2013) 307-313
ramic restoration. Some systems based on non-metal or
all ceramic materials have been developed with excellent
esthetic results [11,12]. Other systems have utilized
metal substructures with better esthetics and strength due
to high noble components of the chosen alloys [13,14].
This investigation selected one of these newer ap-
proaches to the fabrication of metal ceramic restorations,
based on a non-cast metal substructure (Captek, Precious
Chemicals USA Inc., Florida). The fabrication technique
consisted of sintering Au/Pt/Pd-containing wax lamina
onto a refractory die to fabricate a porous structural net-
work followed by infusion of molten gold. The result
was a highly dense, well-fitting metal substructure for
porcelain veneering that, because of the absence of a
chemically compatible oxide layer on the porcelain-bon-
ding substructure, required a mechanically retentive de-
sign and meticulous surface cleanliness as critical fac-
tors in providing fracture resistance and clinical success
of the final units [15]. A ceramo-metal bonder (Capbond,
Precious Chemicals USA Inc., Florida) was applied and
fired over the oxide-free gold copings to form an addi-
tional 25-to-35 μm thin layer. The ceramo-metal bonder
was designed to provide a structure of fine, gold-rich
filaments simulating the optical effect of vital pulp in its
light-scattering qualities. The irregular Capbond interior
structure was expected to also play a role in sustaining
strong mechanical bonds between the porcelain and
metal substructure [16].
Radio Frequency Glow Discharge Treatment (RFGDT)
process as described by Baier and DePalma (1970) is a
method for scrupulously cleaning, surface-activating, and
in some cases sterilizing ceramic, metal, and polymer
substrata [17,18]. RFGDT technique has been well
known for surface cleaning and surface modifications,
widely used in fields of biomedical materials [19,20],
removing superficial contaminants so that contact angles
are decreased and surface energy values are increased,
minimizing potential wetting and spreading problems.
The objectives of this study were to characterize the
porcelain/metal interface after compressive fracture of
these ceramic/metal crowns, and to evaluate whether
RFGDT could be used as a method to promote wetting/
spreading effectiveness and to minimize void inclusions
and fracture cracks at the porcelain-metal interfaces.
2.1. Specimen Fabrication
The crown materials used in this study (Captek) were
applied following manufacturer’s instructions, utilizing
an adhesive layer, two metal-impregnated wax lamina
(one heat-fused over and into the other), and a top-bond-
ing agent for the final porcelain application.
Twelve identical crowns were first fabricated from a
master metal tooth. For six specimens, before the place-
ment of each applied layer, the specimen received
RFGDT using the RFGDT unit (Harrick Scientific Corp.,
Ossining, NY). The other 6 specimens were prepared
without RFGDT. Each specimen was treated in an at-
mosphere of residual air for 2 minutes after the vacuum
pressure had reached a level below 0.1 torr. Medium
frequency discharge (35 MHz) was applied. Immediately
after removal from the vacuum chamber, the specimens
were followed by next procedures to avoid possible re-
contamination from room air. The fabrication of crown
specimens was standardized to control processing vari-
ables. Opaque porcelain and dentin porcelain (Omega
900, Vident, Brea, Calif) were applied using a jig with a
consistent powder/liquid ratio. Final porcelain thickness
was measured and adjusted so that all specimens had the
same dimensions (2.0 mm thickness incisally, 1.5 mm
labially, and 1.2 mm lingually). Surfaces of all specimens
were glazed to remove possible exterior surface flaws.
2.2. Specimen Test
An excessive compressive force was applied to the in-
cisal edge of each crown using an analogue force-gage
and tool with a tooth-cusp-shaped head to induce me-
chanical fracture, as illustrated in Figure 1A. The num-
ber, size, and distribution of interfacial flaws (voids,
cracks) were analyzed from optical microscopic and
SEM inspections of the specimens made with or without
RFGDT treatment (Figure 1B).
The fractured specimens were embedded in epoxy
resin (Epofix, Struers Inc., Cleveland, Ohio) and the
Figure 1. (A) Representation of porcelain mechanical failure
from compressive force; (B) Porcelain fracture mode related to
interfacial flaws.
Copyright © 2013 SciRes. OPEN ACCESS
H. Kim et al. / Open Journal of Stomatology 3 (2013) 307-313 309
resin-mounted specimens were sectioned in coronal
planes through mid-crown areas of porcelain failures
with a cross-sectioning diamond disk (Isomet, Buehler,
Lake Bluff, Ill). Specimens were analyzed with a field
emission SEM (S-4000, Hitachi High Technologies
America Inc., Dallas, Tex) at 20 kV operating in secon-
dary electron imaging mode. SEM images were recorded
at ten evenly spaced intervals around the perimeter of
each cross-section specimen of each of the twelve
specimens (Figure 2). Inspections were made of images
(300× magnification) of each site for voids, cracks, and
other morphological correlates of the fracture zones.
2.3. Statistical Analysis
For statistical analysis, all data (number, location, and
failure pattern of voids and cracks) were converted to
numbers per millimeter of the interfaces examined. Data
for the two groups were compared by independent t-tests,
using a significance level of 0.05, to determine differ-
ences between groups.
It was readily observed during the fabrication of the
specimens that RFGDT of the crowns at each step of
preparation resulted in better wetting properties and a
more even, thinner layer of the bonding agent when ap-
plied to the metal substructure (Figure 3A). Non-
RFGDT specimens revealed poor distribution of the
bonding agent (Figure 3B). All specimens were taken to
total mechanical failure, except one of the RFGDT-pre-
pared specimens that resisted the maximum force (ap-
proximately 500 Newtons) applied manually during the
fracturing process. Mechanical failures included com-
Figure 2. (A) Representative Composite SEM image of cross
sectioned ceramic/metal crown prepared with RFGDT; (B)
Representative Composite SEM image of cross sectioned ce-
ramic/metal crown prepared with non-RFGDT.
Figure 3. (A) Bonding agent application to RFGDT specimen;
(B) Bonding agent application to non-RFGDT specimen.
plete or incomplete cracks, adhesive and cohesive frac-
tures in both RFGDT and non-RFGDT groups.
Visual inspections with the light microscope revealed
gross differences between the two specimen groups. The
RFGDT group showed mainly small fragments, with
failure patterns dominated by microfractures (Figure 4).
The non-RFGDT group of specimens showed larger
fracture zones revealing more exposed metal under the
original porcelain-metal interfaces.
Voids counted during the inspection of each SEM im-
age (total of 120 specimen locations) are enumerated in
Tables 1-3. In the non-RFGDT group, from sixty loca-
tions on the SEM images that were inspected, a total of
88 voids was recorded. Many voids (44% of 88 voids)
were found at the opaque/bonding agent interface zones.
In the RFGDT group, only 48 voids were noted and these
were evenly distributed among the metal/opaque/body
porcelain interfaces of the sixty locations.
Void sizes measured from each SEM image ranged
from 4 to 28 micrometers in the RFGDT group, and 4 to
40 micrometers in the non-RFGDT group.
The total number of cracks observed during SEM im-
age inspections (total of 120 locations) is reported in
Tables 4-6. Among the non-RFGDT group, 50 cracks
were noted, mostly extending to the bulk from long runs
at the metal/ceramic interface. In the RFGDT group,
only 35 cracks were found with smaller fractured frag-
ments than in the non-RFGDT group, shorter crack runs
along the metal/ceramic/interface were found in the
RFGDT group. Since the RFGDT specimen group pre-
Copyright © 2013 SciRes. OPEN ACCESS
H. Kim et al. / Open Journal of Stomatology 3 (2013) 307-313
Table 1. Number of voids at porcelain/metal interface.
Voids/mm of interface
Group Voids Mean
voids/specimen Mean SD
RFGDT 22 3.67 0.150 0.920
non-RFGDT 39 6.50 0.268 0.224
Table 2. Numbers of voids within adjacent opaque layers.
Voids/mm of interface
Group Voids Mean
voids/specimen Mean SD
RFGDT 26 4.33 0.178 0.150
non-RFGDT 49 8.17 0.336 0.164
Table 3. Total numbers of boundary layer voids, RFGDT vs.
non-RFGDT specimens.
Voids/mm of interface
Group Total
voids/specimen Mean* SD
RFGDT 48 8.00 0.328 0.158
non-RFGDT 88 14.66 0.605 0.238
*statistically significant (p < 0.05).
Table 4. Horizontal interfacial cracks, RFGDT vs. non-
RFGDT specimens.
Hor. cracks/mm of interface
Group Hor.
Voids/specimen Mean* SD
RFGDT 3 0.50 0.021 0.034
non-RFGDT 13 2.17 0.089 0.071
*statistically significant (p < 0.05).
Table 5. Vertical (bulk material) cracks, RFGDT vs. non-
RFGDT specimens.
Vertical crack/mm
Group Vertical
cracks/specimen Mean SD
RFGDT 32 5.33 0.219 0.109
non-RFGDT 37 6.17 0.254 0.168
Table 6. Total numbers of cracks, RFGDT vs. non-RFGDT
Group Total
cracks/specimen Mean SD
RFGDT 35 5.83 0.239 0.120
non-RFGDT 50 8.33 0.343 0.158
sented very little horizontal cracking (8.6% of 35 cracks)
along the porcelain/metal interface, microfractures seen
away from the interface suggested a predominantly co-
hesive porcelain fracture pattern. This finding was in
Figure 4. Microfractures with radiant fracture patterns
in non-RFGDT group specimen.
strong contrast to the mainly adhesive failures in the non-
RFGDT group of specimens. In the non-RFGDT group
specimens that revealed more horizontal cracks (26% of
50 cracks) than the RFGDT group, most of this horizon-
tal cracking occurred along the metal-porcelain interface.
To affirm that these finding represented equal lengths
of specimen interfaces, actual inspected boundaries were
measured from all specimen SEM images at 300× mag-
nification. The RFGDT group presented a total length of
24.39 mm of the interfaces examined. The non-RFGDT
group showed a total of 24.27 mm of the interfaces ex-
When all data were converted to numbers of flaws per
millimeter of the interfaces examined, it was clear that
there were significantly fewer total voids per millimeter
at the RFGDT group interfaces (including the porcelain/
metal interface and the opaque layer) than at the non-
RFGDT group (t =2.377, df = 9, p= 0.021). There was,
similarly, a significant difference in numbers of horizon-
tal cracks per millimeter between the groups (t = 2.132,
df = 7, p = 0.035) with more cracks occurring in the
non-RFGDT group. No significant group differences
were found for vertical cracks or total numbers of cracks
per millimeter (p > 0.05), also inspected and counted in
each specimen group.
The aim of this work was to diminish the distribution of
interfacial flaws in metal ceramic crowns when the
RFGDT technique was employed, as illustrated by de-
liberate fracture under an excessive compressive load
applied to the incisal edge (Figure 1A).
Clinicians can face failures of metal ceramic restora-
tions during any step in the fabrication and placement
procedure such as tooth reduction, impression making,
selection and manipulation of dental materials, labora-
tory finishing, and cementation of restorations. It is
speculated that the diminished presence of internal flaws
Copyright © 2013 SciRes. OPEN ACCESS
H. Kim et al. / Open Journal of Stomatology 3 (2013) 307-313 311
will preserve the structural strengths of restorations,
however, the current study’s intent was not to measure
the failure load. Rather, it enumerates two major factors
that enhance the likely clinical success of metal ceramic
restorations. The first factor is the intrinsic material
properties, differentiated into several categories as Ber-
tolotti described [21]. The chemo-mechanical compati-
bility of porcelains and casting alloys plays a key role in
the success of metal ceramic crowns. It is generally ac-
cepted that the three bonding mechanisms of porcelain/
casting alloys are mechanical interlocking, true covalent
chemical bonding, and a physical variant of true chemi-
cal bonding termed Van der Waals bonding.
Thermal expansion/contraction incompatibility is an-
other important factor in metal ceramic restoration fail-
ures. Many studies have shown that failures occur both
during fabrication and in the mouth as a result of thermal
expansion incompatibility. However, thermal expansion/
contraction values alone are not sufficient to predict
thermal expansion compatibility. Thermal history, ge-
ometry of prosthesis, and many processing variables are
equally important. Other intrinsic factors affecting the
strength of metal ceramic restorations are diffusion of
metal oxides into porcelain, surface roughness, and in-
terface reactions such that thermodynamic driving forces
result in porcelain wetting of the metal and spreading of
the molten glass. In previous work, beneficial smaller
contact angles have been obtained only when the metal
surface was roughened, not recognizing that the lower-
than-ideal degree of surface cleanliness and surface en-
ergy was not actually overcome by this process. Here,
RFGDT is proposed to be able to enhance the integrity of
even very smooth bonding surfaces.
The second factor is extrinsic surface flaws produced
by machining, grinding, and other surface treatment
methods. The sizes and numbers of extrinsic surface
flaws, microcracks, porosities and many different in-
traoral variables do affect the strength of metal ceramic
restorations [22].
Dental porcelains are brittle materials, therefore, frac-
tures occur in porcelain when the applied forces produce
stresses at flaw tips equal to the intrinsic tensile strength
of porcelain, as described by Griffith [23]. First, the frac-
ture occurs at the most severe flaw present in the stressed
region. In dental porcelain, surface flaws are the most
important cause of fracture failures [24]. The flaws grow
to critical size when placed under increasing stress. A
chemical reaction between porcelain and water is re-
sponsible for delayed failure in porcelains [25]. The slow
extension of the crack continues until the stress intensity
at the crack tip reaches a critical value for the particular
material. The stress intensity at the crack tip at the time
the crack becomes unstable is a material property, termed
the “critical stress intensity factor” or “fracture tough-
ness”. Ceramic strength is directly related to fracture
toughness and inversely related to the square root of the
flaw size [26]. Flaws can be introduced into a metal ce-
ramic restoration during porcelain powder mixing, build-
up, firing, and later contouring, or they can be inherent in
the microstructures from grain size and thermal-coeffi-
cient mismatches [21]. Kelly et al. analyzed failures of
all-ceramic fixed partial dentures (FPDs) in clinical and
laboratory situations [26]. They reported that failures
originated from either the external surface of connectors
of FPDs or from the core-veneer interfaces. For appro-
ximately 75% of all specimens, crack initiation occur-
red at the core-veneer interface, indicating that the inter-
face is both a location of high tensile stress and an im-
portant locus of structural flaws. Interfaces can be the
sites of unique defects, boundary phases, and thermal
incompatibility stresses due to the elastic modulus mis-
matches across the interfaces. Scanning electron micro-
scopic observations revealed numerous porous defects in
both core and veneer ceramics at the interface. It can be
appreciated that restoration failure was originated from
the ceramo-metal interface. Kelly et al. also emphasized
the importance of flaw size, indicating that calculated
tensile strengths of ceramics were reduced with increases
of the flaw sizes [26]. Therefore, dental porcelain is very
sensitive to flaws in size, number and distribution in the
area of highest tensile stress, and the flaws control the
material’s fracture toughness. In the current work, the
observed size of internal voids near the interface between
the bonding agent and opaque/dentin porcelain ranged
from 4 to 40 micrometers in the non-RFGDT group of
specimens, but only from 4 to 28 micrometers in the
RFGDT group. Because ceramic materials fail as a result
of crack propagation and fracture, this RFGDT-imparted
improvement suggests that its increased use during bond-
ing of ceramic materials can lead to improve strength,
fracture resistance, and improved performance due to a
stronger bond between the porcelain and the metal sub-
When the bonding agent was applied to the metal sub-
structure in this work, it was observed that RFGDT did
promote a more even and thinner layer of that bonding
agent. Non-RFGDT specimens showed poorer distribu-
tion of the bond layer. Also, the non-RFGDT specimens
showed weaker sites of the metal/ceramic restorations,
when taken to total mechanical failure, where gross vis-
ual and light stereomicroscopic inspection showed sub-
stantially larger areas of metal-baring delamination than
those of the RFGDT group which presented smaller
fractured fragments with crushed failure patterns and
fewer areas of metal exposure. Flaws observed at the
electron microscope level were defined as follows: Mi-
crofractures were radiant fracture patterns near the frac-
tured surfaces or crack lines (Figure 4). Voids were
Copyright © 2013 SciRes. OPEN ACCESS
H. Kim et al. / Open Journal of Stomatology 3 (2013) 307-313
smooth-bordered oval-shaped bubbles found on either
the metal substructure or in the opaque porcelain layer
(Figure 5). Cracks were divided into vertical cracks that
extended from the bulk to the metal/porcelain junction
and horizontal cracks were those initiated along the
metal/porcelain interface (Figure 6). Cross-sectional
scanning electron microscope (SEM) images revealed
the existence of internal voids between metal substruc-
ture/opaque/body porcelain interfaces and/or within por-
celain layers. A correlation between the existence of flaw
and porcelain failures has been found from the SEM im-
ages (Figure 7). The total number of voids per millime-
ter at the interfaces in the RFGDT group was signifi-
cantly lower than in the non-RFGDT group. The non-
RFGDT group demonstrated more horizontal cracks per
millimeter than the RFGDT group, the apparent result of
easier porcelain delamination from the metal substruc-
tures. Compression-induced vertical fracture cracks were
similar in the two test groups, confirming that the bulk
material properties had not been modified by the RFGDT
process. Thus, it was found that RFGDT decreased in-
terfacial flaws in general, even though there were not
Figure 5. Small voids at the boundary of the dentin
and opaque porcelain layers in RFGDT group speci-
Figure 6. Vertical and horizontal cracks generated
along the metal/porcelain interface in non-RFGDT
specimen group.
Figure 7. Vertical fracture lines noted in bulk material,
propagating to porcelain-metal interface in a non-
RFGDT specimen.
statistically significant differences in total numbers of
cracks of all types between the two groups. Power analy-
sis, using these already-obtained test results, showed that
a larger number of specimens would be required to es-
tablish significance in this regard (i.e. A total of 19
specimens are needed to determine differences in total
number of cracks between the RFGDT and non-RFGDT
groups with 80% power of achieving statistical signifi-
cance at 10% level). Glow-discharge-treatment increased
the integrity of the tested crowns mainly by diminishing
the number of interfacial voids, and resulted in fewer
horizontal cracks along the metal/porcelain interfaces
when specimens were broken. Improved integrity of ce-
ramic/metal crown interfaces by glow-discharge-treat-
ment should lead to the better clinical performances, based
on these mechanical fracture trials and SEM analyses,
and should better maintain the overall integrity of porce-
lain-metal interfaces during functional loading.
Within the limits of this investigation, it is possible to
conclude the following:
1) Mechanical fracture of ceramic/metal crowns leads
to complete and incomplete cracks as well as adhesive
and cohesive fractures, with differences in distribution of
the initiating sites attainable by surface energy modifica-
tion of the bonding faces during the preparation proc-
2) Glow-discharge-treatment at each step of ceramic/
metal fabrication increases the integrity of ceramo-metal
interface zones by diminishing the numbers of interfacial
3) Improved integrity of ceramic/metal crown inter-
faces, achieved by RFGDT, should result in better clini-
cal performances under higher compressive forces.
The authors thank Precious Chemicals Inc. for donating CaptekTM
Copyright © 2013 SciRes. OPEN ACCESS
H. Kim et al. / Open Journal of Stomatology 3 (2013) 307-313
Copyright © 2013 SciRes.
materials for this study, and Mr. Peter J. Bush in the UB South Campus
Instrument Center for his assistance with Scanning Electron Micros-
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