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Comparison of Fractural Strength of Metal Ceramic Bonding Obtained byMicrowave and Conventional Oven Heating.

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Comparison of Fractural Strength of Metal Ceramic Bonding Obtained by
Microwave and Conventional Oven Heating.
BY
Eiad Elathamna
May 31st, 2011
A thesis submitted to the Faculty of the Graduate School
The State University of New York at Buffalo
In partial fulfillment of the requirements for the degree of Master of Science
Department of Oral Science
UMI Number: 1500470
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UMI 1500470
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Dedication
This thesis is dedicated to my wonderful parents, who have raised me to be the person I am today and
have supported me from the beginning of my studies. Thank you for everything.
This thesis is dedicated to my late wife Abeer, who has been with me every step of the way, through good
and bad times. Thank you for all the unconditional love, guidance, and support that you have always
given me. I love you and always in my heart.
Finally, this thesis is dedicated to my kids Daniah and Omar; you are the joy of my life. I love you both.
ii ACKNOWLEDGEMENT
I would like to thank my family and friends for their encouragement, assistance, support and
collaboration.
I would like to take this opportunity to offer my heart-felt thanks to a number of people without whose
help and support this thesis would not have been possible.
Dr. Edward Monaco Jr., my research mentor and postgraduate program director, who has devoted his time
to this project. Your guidance, support and words of encouragement helped me successfully reach my
goals.
Dr. Hyeong-II Kim for your help, support and being on my committee.
Dr. Jenifer Kuracina I would like to thank you for your advice, support, kindness and serving on my
committee.
Dr. Elaine Davis for being on my committee and helping me with the statistical analysis.
Dr. Carlos Munoz and Dr. Marc Campillo to whom I would like to express my sincere appreciation for
their help and guidance throughout this project.
iii Peter J. Bush for his help with the XPS analysis.
Ivoclar Vivadent Inc/ research and development department for kindly providing the materials and
allowing me to conduct the testing at their facility.
iv Table of Contents
1. Abstract…………………………………………………………………..... ix
2. Introduction………………………………………………………………...... 1
3. Literature Review……………………………………………………………. 4
4. Materials and methods…………………………………………………….....13
5. Results……………………………………………………………………..... 19
6. Discussion ………………………………………………………………….. 24
7. Conclusion………………………………………………………………….. 28
8. References………………………………………………………………….. 29
9. Appendix………………………………………………………………….... 32
v I-
List of Tables
1. Table 1. Composition and physical properties of alloy ………………………... 49
2. Table 2. Composition and physical properties of porcelain……………………..50
3. Table 3. Composition and physical properties of opaque……………………….51
4. Table 4. Descriptive statistic for flexural strength………………………………19
5. Table 5. Multiple comparison results……………………………………………21
6. Table 6. XPS analysis of conventional oven specimen………………………….52
7. Table 7. XPS analysis of microwave oven specimen…………………………...53
vi II - List of Figures
1. Microwave furnace with thermal pod, thermocouple, and controller………. 32
2. Electromagnetic spectrum and microwave range……………………………. 33
3. Conventional and microwave oven heating pattern…………………………. 34
4. Diagram of microwave system………………………………………………. 35
5. Specimen geometry………………………………………………………….. 36
6. Heat pressure condenser……………………………………………………... 37
7. SiC susceptors in position……………………………………………………. 38
8. Specimen mounted on Instron testing machine……………………………… 39
9. Diagram to determine coefficient k………………………………………………... 40
10. Recording microphone adjacent to testing Instron…………………….……. 41
11. Diagram of amplitude vs. time and frequencies………………………..…… 42
12. Diagram of load curve vs. sound curve…………………………………..…. 43
13.Diagram of flexural strength (MPa) for test groups…………………………. 20
14.Specimens embedded in epoxy resin for ESM analysis………………….….. 44
15.XPS analysis of conventional oven specimen ………………………………. 22
16.XPS analysis of conventional oven specimen……………………………….. 23
17.Diagram of conventional oven x-ray diffraction analysis…………………… 45
vii 18. Diagram of microwave oven x-ray diffraction analysis………………….….. 46
19. SEM of inter face of metal ceramic sample sintered in microwave oven……. 47
20. SEM of inter face of metal ceramic sample sintered in conventional oven…. 48
viii Abstract
Objectives: The goal of this study was to evaluate the bond strength of porcelain fused to metal sintered
in a microwave oven and compare it to that sintered in a conventional oven.
Methods: High gold alloy (d.SIGN 91, Ivoclar Vivadent) was cast into 30 (25x3x0.5mm) strips
following manufacturer’s directions. An 8x3x1 mm porcelain strip (d.SIGN, Ivoclar Vivadent) was
centered on each of the metal strips and sintered as described below. The specimens were randomly
divided into three groups (n=10). The first group (control) was degassed and sintered in a conventional
oven. The second group was oxidized in a conventional oven and sintered in a microwave oven. The third
group was oxidized and sintered in a microwave oven. All specimens were subjected to a three-point
bending test at a crosshead speed of 1.5mm/min-1 until debonding occurred, using a universal testing
machine. A precision measurements microphone was used to assist in ascertaining the point in time when
the initial debond/crack occurred. Fractured specimens were carbon coated and examined with scanning
electron microscope for fractographic analysis. Data were statistically analyzed using one way ANOVA
and the Tukey HSD test was used for all pair wise multiple comparisons (Į=.05) using statistical software
SPSS 16.0.
Results: The mean debonding strength values in MPa for each group were: 69.51 (SD 5.16) for the
conventional oven process, 55.25 (SD 10.6) for combined conventional degassing and microwave
sintering, and 61.16(SD 10.99) for microwave processing. Significant differences among the groups were
ix detected by one-way ANOVA (p=0.008). Post-hoc analysis using Tukey test revealed significant
differences between the conventional and the combined processes (p=0.006). No statistical differences
were detected between the conventional and the microwave processes (p=0.132) or between microwave
and the combined processes (p=0.348).
Conclusions: The fractural strength of sintered porcelain fused to metal sintered with a microwave is
comparable to that sintered in a conventional oven.
x Introduction:
Microwave sintering technologies have been developed and traditionally used for industrial processing of
polymers, rubber, wood, and ceramics. Processing of industrial ceramics with microwave energy was first
explored in the 1950’s and investigated in the 1960’s by Tinga et al [1] . Recently this technology has
been utilized for processing biomaterials, including dental ceramics. Only a few investigators have
studied the feasibility of utilizing microwaves for sintering bioceramics, metals and metal ceramic
structures[2, 3].
Microwave heating is fundamentally different from the conventional process. In conventional firing, heat
is applied to the external surface of the ceramic structure and transmitted to the core by thermal
conduction. This results in high temperature gradients and residual stress within the ceramic material[4].
In contrast, microwave heating generates heat both internally and externally within the material resulting
in volumetric heating. The thermal gradient and flow of heat in the ceramic body are the reverse of those
on conventional heating. This allows heating of small and large samples uniformly and rapidly and
produce fewer thermal stresses[5].
Microwave heating has several benefits over conventional sintering. These benefits include more precise
and volumetric heating, faster ramp up temperature, lower energy consumption and improved properties
of the ceramic materials[6]. Microwaves interact with materials at the molecular level, through molecular
1 interaction with the electromagnetic field. With respect to dental alloys and dental ceramic materials,
microwaves interact with two materials, the first being the microwave interaction with dental alloy,
followed by the interaction of the microwave with dental porcelains. Many investigators believe that
metals are not microwave friendly but recent scientific evidence has demonstrated that microwaves can be
successfully utilized to heat metal powders into solid forms and metal solid structures, provided certain
principles are followed[2].
Patterson et al. compared the cost of microwave and conventional electric furnace heating and concluded
that microwave heating results in an energy saving of as much as 90% over conventional oven sintering
techniques[7].
A simple microwave oven was custom designed by EPL Ceramic Materials (Youngstown, NY), to study
ceramic sintering. It enabled raising the temperature inside a microwave oven to above 1600°C. This
custom built microwave, called ThermWave (Fig 1), incorporated a water jacket to cool the system and
was connected to a circulating water supply. A pyrometer/ thermocouple were inserted from the top of the
oven to accurately determine the temperature inside the oven and a controller box was attached to the
microwave to control the temperature precisely inside the chamber. The microwave operated at a
frequency of 2.45 GHz with a power output capability of 1.25 Kw.
2 Although the superior qualities of microwave technology are common knowledge in the industrial setting,
the effect of microwave sintering on dental porcelain and metal ceramic has been minimally investigated.
Microwave sintering of ceramics is known to result in finer grain size and greater toughness (strength)
with decreased cost[5]. This study was designed to investigate the effect of microwave sintering on the
strength of metal ceramic bond.
3 Literature Review
Microwave sintering:
Microwave is a term given to electromagnetic radiation ranging from 1m to 1mm in wavelength
that corresponds to frequencies of approximately 1 to 300 GHz. This frequency range falls just
below visible light on the electromagnetic spectrum (Fig 2).
The Federal Communications Commission has allocated 915 MHz, 2.45, 5.85 and 20.2-22.2 GHz
for industrial, scientific and medical use. The two most commonly used frequencies are 915 MHz
and 2.45 GHZ[8]. Recently, microwave furnaces that allow processing at variable frequencies
from .9 to 18 GHz have been developed for material processing [9].
Within the past 20 years, the microwave oven has become an essential appliance in most kitchens.
Faster cooking time and energy saving over the conventional cooking methods are the primary
benefits. Although the use of microwaves for cooking food is widespread, the application of this
technology to the processing of materials is a relatively new development. The use of microwave
energy for processing materials has the potential to offer similar advantages in reduced processing
times and energy savings [10].
4 Microwave sintering technologies have been developed and traditionally used for industrial
processing of polymers, rubber, wood, and ceramics. Processing of industrial ceramics with
microwave energy was first explored in the 1950’s and investigated in the 1960’s by Tinga et
al.[1]. Activity in this field began to accelerate in the mid-1970s because of a shortage of natural
gas. During this period microwaves were investigated by Sutton and others for drying and firing of
castable alumina ceramics[5]. Berteaud and Boda (1976) were the first to report achieving
ceramic densification with microwave energy[11]. Bennet et al. sintered a small sample of
alumina in a microwave and compared the sintered density with that obtained at the same
temperatures and in the same amount of time as in conventional oven. They found that the same
density to be achieved with a reduction in sintering temperature of 175-200 °C over the same
sintering time. There was strong evidence that significant enhancement in sintering results can be
achieved by microwave oven [12].
Many different physical phenomena are involved during the microwave processing of ceramics.
The interaction between microwaves and matter takes place through the electric field vector and
magnetic field vector of the electromagnetic field of the microwave and involves polarization and
a conduction process. Various absorption mechanisms have been identified in the interaction of
microwave with matter, including dipole reorientation, conduction of space and ionic charge,
which are primarily found in insulator or dielectric materials[6].
5 The nature of the interaction between microwaves and ceramic is complex and is dependent on the
dielectric property of the ceramic. The dielectric property of a material is its ability to
continuously hold electrons at a high voltage. Such material supports an electric field with
virtually no current passing through it. Ceramics are said to have this property. When microwaves
penetrate the ceramic material, microwave energy is propagated due to the generation of an
internal electric field[6]. This induces translation motion of free and bound electrons. Due to the
excellent dielectric property of ceramics, the translation motion of the electrons is resisted, thereby
causing loss and attenuation of the electric field. This loss results in the production of heat ,
resulting in volumetric heating of the ceramic sample[6].
Microwave sintering is fundamentally different from conventional oven sintering (Fig. 3).
In conventional firing, heat is applied to the surface of the ceramic and reaches the core by thermal
conduction, producing high temperature gradients and stress. In microwave processing the ceramic
is heated both internally and externally[6] . In addition to the heat generated within material,
susceptors made of silicon carbide or molybdenum silicate are placed around the ceramic material
to heat the sample externally by thermal conduction (Fig. 4). As a result of this internal and
external volumetric heating, the thermal gradient and flow of heat in the ceramic body is evenly
applied. This makes it possible to heat small and large samples uniformly and rapidly, with less
thermal stress[2].
6 Many ceramic materials do not absorb microwaves well at room temperature. Susceptors (Fig.4)
help increase the temperature until the dielectric loss in the ceramic is high enough that the
ceramic couples directly with the microwave field[13]. For example, when using silicon carbide
susceptors, zirconia will heat primarily by radiation from the silicon carbide until temperature
reaches approximately around 600°C, at which point the zirconia couples preferentially and heats
volumetrically[13].
Initially, success in microwave heating and sintering was confined mainly to oxide and non-oxide
ceramics, Walkiewicz et al. exposed a range of materials, including six different metals partly
oxidized in air, to a 2.4 GHz field and reported a modest heating but no sintering , with
temperatures ranging from 120°C for Mg to 768°C for Fe[14]. Nishitani reported sintering of
tungsten carbide-cobalt composite by adding a small amount of electrically conducting powders
such as aluminum. The heating rate of refractory ceramic was considerably enhanced. No mention
was made of the microwave sintering of pure metal powders[15].
Until recently, it was believed that metals were not microwave friendly materials. Based on the
available knowledge, metals were considered only to reflect microwaves from their surface(s).
This reflection is due to a skin effect as a result of an induced eddy-current field in the outermost
layers of metal. Penetration depth of the microwaves decreases exponentially due to this effect. In
addition, arcing occurs as a consequence of the build-up of electrons at the corners or edges of the
metal.
7 Roy et al. (1999) described the microwave sintering of powder metals to full density. They were
able to sinter a wide range of standard powdered metal using a 2.4 GHz microwave field, yielding
dense products with better mechanical properties than those obtained by conventional heating.
Roy reported the use of an alumina tube surrounded by ceramic fiber insulation to preserve the
heat inside the tube cavity. Susceptors rods made of silicon carbide or molybdenum silicate
provides an external heat source. Inside the microwave temperature was determined by optical
pyrometers and/or sheathed thermocouples placed very close to the surface of the sample[2] (Fig
1).
The advantage of improved mechanical properties through microwave sintering is mostly due to
uniform volumetric heating and extremely rapid heat up rates with shorter sintering cycles[5].
Uniform heating results in more uniform crystalline structure. In addition, uniform volumetric
heating minimizes thermal heat up stresses and allows for sintering of large and/or more complex
shapes[5].
Harmer and Brook have reported that rapid sintering produces a fine grain microstructure. By
heating a specimen quickly to high temperature, grain growth, a dominant process at low
temperature, is thus minimized[16]. Patterson et al reported a slight increased toughness of
alumina which they attributed to the smaller grain size attained by microwave sintering[7].
8 Holcombe et al. reported a greater thermal shock resistance in the form of higher fracture stress
values for microwave sintered Yttrium-stabilized zirconia (2 % wt.Y2O3-ZrO2). He theorized that
there was localized melting at the surface of pores that glaze the pore surface and put the region
surrounding the pore in localized compression[17].
Prasad et al reported the advantage of improved properties of dental ceramics by comparing the
surface roughness and flexural strength achieved by glazing porcelain specimens in conventional
and microwave ovens. The result of their investigation showed superior surface results of
microwave glazing compared to conventional oven glazing for the two different porcelain
evaluated[3].
Microwave sintering is also fuel efficient. It has been shown that conversion of a fossil fuel to
electricity is about 30% efficient, conversion of electricity to microwave is about 50% efficient
and the conversion of microwave into heat is about 80% efficient[5]. The conversion of fossil fuel
to heat during a typical sintering operation is about 40% efficient[5]. Patterson et al. compared the
cost of microwave and conventional electric furnace sintering and concluded that microwave
sintering results in an energy saving of as much as 90% over conventional sintering techniques[7].
9 Oxide layer and Porcelain-Metal bonding:
Metal ceramic restorations are widely used in restorative dentistry because of their strength,
esthetics, durability and insolubility in oral fluids. The poor tensile, shear and impact strength of
dental porcelain has been partially overcome by fusing it to a cast alloy substructure. Certain
physical and chemical requirements must be fulfilled to obtain satisfactory porcelain fused to
metal bond[18].
Bonding porcelain to metal supports the porcelain with a metal substructure. It increases the
strength of porcelain by reducing ceramic surface defects at the porcelain-metal interface , and
desirably places the external surface of porcelain in compression[19].
Bonding is thought to result from mechanical interlocking, van der Waals forces, chemical
bonding and compressive forces. While the contribution to the various bonding mechanism has
been debated, chemical bonding has been described as the primary mechanism for metal ceramic
bonding[20].
Acceptable metal-ceramic bonding requires alloy and porcelain to be chemically, thermally and
mechanically compatible. Chemical compatibility implies a bond strong enough to resist both
transient, residual thermal stresses and mechanical forces encountered in clinical function.
10 Thermal compatibility requires a fusing temperature of porcelain that does not cause distortion of
the metal substructure, along with close match of coefficient of thermal expansion with metal
[20].
Chemical compatibility implies formation of a strong bond during porcelain firing. It initially
involves the formation of an adherent oxide bound to the alloy and then, the ability of the oxide
layer to saturate the porcelain, completing the chemical bond of porcelain to metal. The degassing
or oxidation firing produces the oxide layer required for bonding. Oxidation firing time and
temperature must be sufficient to produce an adequate oxide layer for bonding. Excessive
oxidation results in a nonadherent oxide layer and poor metal ceramic bond[21].
Dent et al. investigated the effect of vacuum on the formed oxide layer. They examined the effect
of vacuum and its absence on oxidation cycles and bond strength. They found no difference in
the bond strength caused by the absence or utilization of vacuum firing [21].
To achieve a sound chemical bond to ceramic, a substrate metal has to be able to form a thin,
adherent oxide layer, one that is preferably light in color for aesthetics. Stresses that develop in the
ceramic adjacent to the metal/ceramic interface can enhance the fracture resistance of the metal
ceramic bond if the stresses are mainly compressive in nature. They can increase the susceptibility
of crack formation if they are predominantly tensile in nature[22].
11 Bond strength tests measure both resistance to applied stress and residual stress alike. Several
geometries have been used to evaluate bond strength, including testing in tension, shear, three and
four point bending test. Investigators have demonstrated differences in bond strength among
dental alloys for porcelain veneering. This difference demonstrates the importance of the oxide
layer in porcelain to metal bonding[20].
Purpose
The purpose of this research is to evaluate the flexural bond strength (three point bending test) of
porcelain fused to metal sintered in a microwave oven and compare it to porcelain fused to metal
sintered in a conventional oven.
Research Hypothesis
The flexural bond strength of microwave oven sintered porcelain fused to metal is comparable to
that of conventional oven sintered porcelain fused to metal.
12 Materials and Methods
ISO 9693(e), Metal –Ceramic Dental Restorative Systems, specifies the procedure for
characterizing the debonding strength of metal –ceramic dental restorations. The material
requirements and testing methods of ISO9693 were followed for this investigation[23].
A high noble alloy (d.SIGN 91, Ivoclar –Vivadent, Inc, Amherst, NY) was used. The composition
of the alloy in weight percent is found in table I. Thirty wax patterns were made from 22-gauge
casting wax ( Green Casting Wax; Corning Rubber Co, Brooklyn, NY) and cut into 25 × 3× 0.5
mm ( l× w × h ) flat strips . The wax patterns were sprued and invested with a phosphate-bonded
investment (Sure-Vest High Heat; Ivoclar-Vivadent, Inc).
Alloy was melted in an individual ceramic crucible with a multi orifice gas-oxygen torch and cast
with a standard broken-arm machine (Centrifico Casting Machine; Kerr Corp, Orange, Calif). All
castings were bench-cooled to room temperature, divested and cleaned by airborne –particle
abrasion using 50-µm Al2O2 (Williams Blasting Compound; Ivoclar Vivadent, Amherst, NY) at 4bar pressure. All metal bars were ultrasonically cleaned in distilled water for 10 minutes.
13 The 30 bars were adjusted to 25 × 3× 0.5 mm prescribed by ISO 9693 (Fig. 5), using aluminum
oxide barrel stones and a laboratory hand piece (Z 500 Lab Motors; NSK, Kanuma, Japan).
Dimensions were verified with a Boley gauge (Hu-Friedy, Chicago, IL).
The bars were randomly divided into three groups (n=10/group). The first group (control) was
oxidized and sintered in a conventional oven (Programat P300; Ivoclar, Vivadent, Amherst, NY).
The second group was also oxidized in a conventional oven under vacuum (Programat P300;
Ivoclar, Vivadent, Amherst, NY) and sintered in a microwave oven (ThermWave 1.3; EPL
Ceramic Materials, Youngstown, NY). The third group was oxidized and sintered in a microwave
oven (ThermWave 1.3).
Metal strips, ultrasonically cleaned in distilled water, were suspended on a firing platform to
simulate the standard level of a crown framework within the oven muffle and oxidized according
to manufacturer’s instructions. IPS d.SIGN porcelain (Ivoclar- Vivadent, Inc, Amherst, NY), was
used to prepare the metal-ceramic specimens, the composition and physical properties of porcelain
are found in tables 2&3.
Two layers of opaque porcelain with a total thickness of 0.1 mm were applied over a rectangular
area of 8×3 mm in the center of one side of each cast metal specimen strip. Two layers of body
porcelain were applied to metal strips specimens, vibrated and condensed to produce minimal
14 shrinkage. Metal-ceramic group two and three sintered in microwave were placed in a heat
pressure condensation machine (IVOMAT IP3; Ivoclar, Vivadent, Amherst, NY), to minimize the
amount of air incorporated in porcelain since microwave oven sintering has no vacuum (Fig 6).
Specimens were trimmed and polished with cylindrical diamond rotary instruments (Brasseler
USA, Savannah, GA). Measurements were made of the porcelain addition using a Boley gauge
(HU-Friedy) in multiple locations to ensure a flat and rectangular shape porcelain having a
thickness of 1.1± 0.1 mm. Final glaze firing was performed according to the recommendations of
manufacturers. Metal-ceramic debonding strength was determined using the Schwickerath crack
initiation test according to ISO 9693:1999(E). All specimens were subjected to a three-point
bending test at a crosshead speed of 1.5mm/min-1 using a universal testing machine until
debonding occurred, ( Model 4204;Instron Corp , Canton ,Mass) (Fig 8) . The debonding /crack
initiation strength for material loaded in a 3- point flexure test configuration is determined by:
Tb= k× Ffail
Where Tb is the debonding strength /crack initiation strength in MPa; the coefficient K is a
function of the elastic modulus of the metal used and its thickness, which is determined from a
table contained in ISO 9693; 1999(E) (Fig 9). The k value used was 4.68 MPa/N and Ffail is the
load to failure.
15 The 3-point bending apparatus had a support span of 20.0 mm. A center load was applied at a
speed of 1.5mm/min-1 with a 5 KN load cell used for all testing. All flexure bars were placed so
that the porcelain layer was opposite the applied load. Load versus crosshead extension curves
were generated for each specimen.
To help determine initial metal-ceramic debonding and crack initiation a precision measurements
microphone (Model M53; LinearX Systemic, Tualatin, OR) was placed close to the test specimen
( Fig 9 ), and an amplitude –versus time graph was generated using noise analysis software (
pcRTA Version 2.30 ; LinearX System Inc). The 5 and 10 kHz frequencies were evaluated
because these frequencies was sensitive to noise generated during metal-ceramic debonding (Fig
10) [24].
The pink noise generator was selected and an American National Standards Institute (ANSI-A)
weighted filter was used with the dynamic range fixed between -60 and +20 dBm. Pink noise is
the most common source used during noise analysis[24]. The selection of the ANSI-A weighted
filter helped attenuate low frequencies while allowing higher frequencies to pass relatively
unattenuated. Noise analysis was started simultaneously with the flexure test so that there would
be a time correspondence between the amplitude-versus time curves and the load versus-crosshead
extension curve (Figs 11&12).
16 Following testing each specimen was observed by light microscope at X20 to X80 magnification
and 3 flexure bars from each group were chosen to b e evaluated by scanning electron microscopy
(SEM) operated at 25 KV (Hitachi SU70, Schaumburg, USA). The specimens were sputter-coated
with carbon and examined with SEM. Secondary electron images showing surface topography of
both metal and porcelain of each specimen were made.
Two extra specimens , one representing the conventional oven and the second representing the
microwave oven, were sectioned and embedded in epoxy resin to be viewed by SEM to evaluate
the interface between the metal substrate and the overlying porcelain in both samples(Fig 13).
In addition, two extra metal strips were cast, one oxidized in the conventional oven and the second
sample oxidized in the microwave oven. The two samples were evaluated under X-ray
Photoelectron Spectroscopy (XPS), also known as ESCA (electron spectroscopy for chemical
analysis) to analyze and evaluate the oxide layer on the metal strips.
Surface chemistry of the formed oxide layer was analyzed using SSX model 100 small spot XPS (
Ultima IV, Rigaku, Shibuya-Ku, Japan) with monochromatized Al KĮ radiation. Different depths
were analyzed by Argon ion sputtering of the sample surfaces.
17 Although the actual voids and surface irregularities were not measured from the SEM images
obtained, the overall appearance in terms of numbers and distribution of voids and other surface
irregularities were qualitatively evaluated from ESM.
Data were statistically analyzed using one way ANOVA followed by Tukey post hoc test at
significance level Į=0.05 using statistical software SPSS 16.0 (SPSS Inc., Chicago, USA).
18 RESULTS
The mean flexural strength (MPa) and standard deviation for each group are listed in
Table 4.
Table 4: Descriptive statistics for flexural strength (MPa) (n=10/group).
Mean
Conventional
(Control)
Conventional-Microwave
(Combined)
Microwave
69.5
55.25
61.16
5.17
10.86
11.68
SD
All specimens for all three groups achieved a mean debonding strength/ crack initiation strength greater
than 25 MPa and met requirements set by ISO 9693: 1999(E) for metal ceramic system table 5. The
results for flexural strength testing are illustrated in (Fig 14).
19 80
Flextural Strength (MPa)
70
60
50
40
30
69.51
(5.17)
61.40
(11.67)
20
55.25
(10.68)
10
0
Conventional
Microwave
Combined
Figure 14: Flexural strength (MPa) by groups.
None of the flexure bars failed from crack initiating in the porcelain on the tensile surface, but rather each
of the bars failed at the metal ceramic junction at one end of the porcelain addition margin. SEM and light
microscope examination of representative samples of all experimental groups showed ceramic still
attached to metal. SEM and light microscopy images showed that metal was also exposed, indicating a
mixed mode of cohesive/adhesive failure.
Flexural strength data were analyzed using one-way ANOVA at a significance level of 0.05. A
statistically significant difference between the groups was demonstrated (F2, 27=5.88, p=0.008).
20 Follow up multiple comparisons post-hoc analysis using Tukey HSD test revealed significantly higher
mean flexural strength for conventional processing group than the combined processing group (69.5 VS
55.25 MPa ; p=.006). There was no statically significant difference between microwave group (61.16
MPa) and either conventional group (p=0.132) or combined group (p=0.348) .Homogeneous subsets for
these groups are illustrated in Table 5.
Table 5: Homogeneous subset for groups
Tukey HSD
Bond Strength (MPa)
Subset for alpha = 0.05
Group
conventional and microwave(combined)
microwave
conventional
N
1
10
55.250
10
61.160
10
Sig.
61.160
69.510
0.348
21 2
0.132
A larger sample size may have resulted in a finding of a statistically significant difference in bond
strength between conventional oven group and microwave group. However, the bond strength of the three
groups was more than the minimum clinical requirement of 25MPa set by ISO 9693E.
The XPS analysis at the surface 0A°, 20 A° and > 200 A° depth, shows almost no difference in the
composition and percentage of the oxide layer formed on both conventional and microwave samples as
shown in (Figs13,14).
100%
90%
80%
70%
60%
Ga
50%
In
40%
Pd
Au
30%
20%
10%
0%
0 A°
20 A°
> 200 A°
Figure 15: XPS analysis of Conventional oven specimen.
22 100%
90%
80%
70%
60%
Ga
50%
In
Pd
40%
Au
30%
20%
10%
0%
0 A°
20 A°
> 200 A°
Figure 16: XPS analysis of Microwave oven oxidized specimen.
23 Discussion
This study investigated the bond strength of a metal ceramic system sintered in both conventional
and microwave ovens using a three point bending test (ISO9693). The rationale for using the
metal alloy d.SIGN 91 is its standard usage in dentistry. The Porcelain IPS d.SIGN was used
because of its compatible thermal expansion coefficient and high strength.
To date, many investigators have studied bonding of porcelain to metals. Four types of bonding
are thought to be involved, namely, chemical bonding, mechanical interlocking, compressive force
and van der Waals forces. Chemical bonding, however, has been recognized by the majority of
investigators to be the principal mechanism for porcelain metal bonding [18, 20, 25].
In 1959, King et al proposed the interface saturation theory, which states that the bond is formed
directly between the substrate metal and a layer phase of the porcelain. Pask and Fulrath (1962),
found that a monomolecular layer of oxide is present between the oxide saturated glassy phase and
metal[18].
The effect of vacuum on the formed oxide layer has also been investigated. Dent et al. evaluated
the bond strength of porcelain fused to metal under different oxidation cycles. They used three
groups oxidized under vacuum with different holding times, and three groups oxidized under no
24 vacuum with different holding time. Results showed no difference in bond strength among the
groups. The group that showed the highest bond strength was the group oxidized under no vacuum
with the longest holding time[18].
To investigate oxide layer formation in both the conventional oven and the microwave oven and
its effect on bond strength. XPS analysis of the metal samples from both ovens was done at
different depths, at the surface (0 A°), 20 A° and > 200 A° (Figs 17&18). The results revealed
general similarities in the composition of the oxide layer, as shown in table 5&6.
XPS analysis showed that upon heating of metal samples Indium (In) and Gallium (Ga) tend to
migrate toward the surface, showing higher concentration in both conventional and microwave
ovens. The concentration of these elements is reduced at deeper levels, indicating surface
segregation.
During the three point bending test, all samples exhibited a progressive debonding from the
terminal site of one end of the porcelain ceramic interface towards the other end of the specimen.
This is consistent with the stress analysis reported by Anusavice et al. indicating that the tensile
forces on the metal ceramic bond are the greatest at the ceramic termination sites. Furthermore,
SEM and light microscopy examination of representative samples of all groups showed ceramic
still attached to metal[26, 27].
25 Using the precision measurement microphone system, it was noted that the maximum recorded
load did not correspond with initial metal ceramic debonding recorded by the Instron, but rather to
initial debonding, followed by delaminating of metal and ceramic until an increase in load could
not be sustained. Initial metal ceramic debonding did correspond with the point on load versus
extension curve where the relationship was no longer a straight line. Failure sequence begins with
the nonelastic behavior or plastic deformation in the metal ceramic complex. Loads corresponding
with initial metal ceramic debonding recorded by using the measurements microphone were lower
than the peak load observed on the Instron machine load versus displacement curve. All three
groups recorded considerably higher bond strength than 25 MPa, the minimum required by ISO
specification.
During SEM examination of the embedded epoxy resin sectioned specimens, conventional oven
and microwave samples showed a comparable metal ceramic interface, though there was evidence
of porosities on both porcelain and the metal surface. There were more on the porcelain of the
microwave sintered sample, although this observation was not measured. This could be attributed
to the fact that microwave sintering was not done under vacuum. The use of the heat pressure
condensation machine was intended to offset this, although this observation is merely anecdotal.
26 Carr and Brantley indicated that during casting procedures liquid palladium can absorb large
quantities of gases. These gases, in turn, are released during the solidification process and must be
accommodated by a coarse grained investment materials or microscopic porosity may occur in the
casting. Also they contended that a rough metal surface is capable of trapping air pockets and
contaminants, which may lead to gas formation during ceramic firing, causing porosity production
in the ceramic. In addition, palladium oxides are unstable and dissociate at elevated temperature,
possibly creating oxygen bubbles at the interface. This might contribute to some of the air bubbles
seen on metal ceramic interface under SEM.
Although there was a significant difference between the first group (conventional oven) and the
second group all bond strength values were above the ISO standard of 25MPa.
27 Conclusions Based on the results of this study, and with the particular type of alloy and porcelain combination that
was used, it can be concluded that fractural strength of metal ceramic specimens obtained by the
microwave oven were comparable to the fractural strength of those sintered in the conventional oven. The
microwave not only has the potential to save time and energy, but also appears to produce metal ceramic
specimens with satisfactory mechanical and microstructure properties. The microwave oven displayed the
ability to be used in sintering metal ceramic with the ability to control temperature and time in the
different cycles required in the process. The absence of a vacuum in our microwave did not affect the
specimen’s bond strength.
28 REFERENCES
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31 Appendix
Thermocouple
Thermal pod
Controller box
Figure: 1 Microwave furnace (MRS ThermWAVE Model II) with thermal pod, thermocouple, and controller.
32 Figure2: The electromagnetic (EM) spectrum and the microwave range. The most commonly used
frequency is 2.45GHz with a wavelength of 12.2 cm. Courtesy of Clarck D.E, Folz, D.C.
33 Conventional
Microwave
Figure: 3 heating patterns in conventional oven and microwave furnace.
Courtesy: Sutton W. Microwave Processing of Ceramic Material- Ceramic Bulletin,
1989; 68(2):376-386.
34 Figure: 4 Diagram of microwave system for sintering of powdered-metal parts
The system consists of a 2.54-GHz microwave oven (cavity) and a ceramic (alumina) tube which is
inserted into the cavity and surrounded by ceramic fiber blocks. Inside the insulation, SiC or MoSi2
susceptors rod are inserted. The sample is placed inside the ceramic tube. The system is capable of
achieving temperature up to 1,600 °C, and any desired atmospheres (such as H2, N2, Ar) can be used. The
same system has been used with or without susceptors (SiC/MoSi2) rods; sintering times without the rods
are somewhat longer .courtesy: Ruston Roy DA, Jiping Cheng & Shalva Gedevanishvili. Full sintering of
powdered-metal bodies in a microwave field. Nature. June 1999; 399, 668 – 670.
35 Figure 5: Specimen Geometry.
36 Figure 6: Heat Pressure Condenser IVOMAT IP3 IVOCLARE.
37 SiC Susceptors
Thermal pod
Metal ceramic specimens
Fig. 7 Figure showing SiC susceptors in position inside a microwave.
38 Figure 8: Instron testing machine.
39 Figure 9: Diagram to determine the coefficient k as a function of metal substrate thickness dM and
Young’s modulus EM of the metallic material.
40 Figure 10: Recording microphone adjacent to testing Instron.
41 Figure 11: 5 KHz and 10 KHz frequencies were measured over time.
42 Figure 12: Load curve verses sound curve shows initial debonding/cracking of porcelain.
43 SECTIONED SPECIEMEN
Figure 14: specimens embedded in epoxy resin for ESM analysis.
44 Figure 17: Conventional oven sample x-ray diffraction analysis at surface (0 A°).
45 Figure 18: Microwave oven sample x-ray diffraction analysis at surface (0 A°).
46 Interface of metal ceramic
Figure 19: SEM of inter face of metal porcelain sample sintered in microwave oven.
47 Interface of metal ceramic
Figure 20: SEM of inter face of metal porcelain sample sintered in conventional oven.
48 Table 1: Composition and physical properties of d.SIGN 91.
Composition
Color
0.2% Offset Proff Stress
Au 60%, Pd30.55% , Ga 1.0%, In 8.4%, Re < 1.0%, Ru < 1%.
White
520 MPa
Vickers Hardness
280
Elongation
31%
Modulus of Elasticity
Density
108,000 MPa
14.3 g/cm2
Range of Fusion
1140-1335°C
Casting Temperature
1360-1420°C
CTE 25-500C
14.2
CTE 20-600C
14.4
49 Table2: Composition and physical properties of d.SIGN porcelain.
Composition
SiO2 50-65%, Al2O3 8-20%, Na2O 4-12%, K2O 7-13%,CaO
7-13%, P2O5 0.0-5%, F 1-3%, Pigments 0-3%
Flexural strength
80± 25 MPa
Chemical solubility
< 100 µg/cm2
CTE (25-500)
Transformation
temperature
12.0± 0.5
510± 10 °C
50 Table3: Composition and physical properties of d.SIGN opaque.
Composition
Flexural strength
Chemical solubility
CTE (25-500)
Transformation
temperature
Al2O3 8-12%, Na2O 2-6%, K2O 5-10%, SiO2 30-40%, ZrO2
15-40%, Glycole 25%, Pigments 25%.
>100 MPa
< 100 µg/cm2
13.6 ± 0.5
600 ± 10 °C
51 Table 6: XPS analysis of conventional oven specimen.
0 A°
20 A°
> 200 A°
Au
31.6%
38.4%
49.4%
Pd
16.8%
21.5%
24.6%
In
44.8%
34.8%
22.4%
Ga
6.6%
5.4%
4.6%
52 Table 7: XPS analysis of Microwave oven specimen.
0 A°
20 A°
> 200 A°
Au
24.7%
40.4%
50.7%
Pd
24.1%
26.5%
28.0%
In
43.6%
29.2%
18.0%
Ga
7.6%
3.9%
53 3.3%
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