Diminished fracture initiation sites in ceramic layers bonded to glow-discharge treated substructure

doi:10.4236/ojst.2013.36051

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Open Journal of Stomatology, 2013, 3, 307-313 OJST

http://dx.doi.org/10.4236/ojst.2013.36051 Published Online September 2013 (http://www.scirp.org/journal/ojst/)

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: *[email protected]

Received 10 June 2013; revised 10 July 2013; accepted 31 July 2013

which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

ABSTRACT

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

1. INTRODUCTION

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.

OPEN ACCESS

H. Kim et al. / Open Journal of Stomatology 3 (2013) 307-313

308

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. MATERIALS AND METHODS

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.

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.

3. RESULTS

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-

H. Kim et al. / Open Journal of Stomatology 3 (2013) 307-313

310

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

Mean

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.

cracks

Mean

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

Mean

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

specimens.

Crack/mm

Group Total

cracks

Mean

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-

amined.

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.

4. DISCUSSION

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

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-

structure.

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

H. Kim et al. / Open Journal of Stomatology 3 (2013) 307-313

312

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-

men.

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.

5. CONCLUSIONS

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-

esses.

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

voids.

3) Improved integrity of ceramic/metal crown inter-

faces, achieved by RFGDT, should result in better clini-

cal performances under higher compressive forces.

6. ACKNOWLEDGEMENTS

The authors thank Precious Chemicals Inc. for donating CaptekTM

H. Kim et al. / Open Journal of Stomatology 3 (2013) 307-313

313

materials for this study, and Mr. Peter J. Bush in the UB South Campus

Instrument Center for his assistance with Scanning Electron Micros-

copy.

OPEN ACCESS

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