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Article
Effect of Lithium Disilicate Reinforced Liner
Treatment on Bond and Fracture Strengths of
Bilayered Zirconia All-Ceramic Crown
Yong-Seok Jang 1,† , Hyeong-Rok Noh 1,† , Min-Ho Lee 1 , Myung-Jin Lim 2, * and Tae-Sung Bae 1, *
1
2
*
†
Department of Dental Biomaterials, Institute of Biodegradable Materials, BK21 Plus Program,
School of Dentistry, Chonbuk National University, Jeonju 54896, Korea; [email protected] (Y.-S.J.);
[email protected] (H.-R.N.); [email protected] (M.-H.L.)
Department of Conservative Dentistry, School of Dentistry, Chonbuk National University,
Jeonju 54896, Korea
Correspondence: [email protected] (M.-J.L.); [email protected] (T.-S.B.);
Tel.:+82-63-250-2120 (M.-J.L.); +82-63-270-4041 (T.-S.B.)
These authors contributed equally to this work.
Received: 30 November 2017; Accepted: 3 January 2018; Published: 5 January 2018
Abstract: This study was performed to evaluate the effect of a lithium-disilicate spray-liner
application on both the bond strength between zirconia cores and heat-pressed lithium-disilicate
glass-ceramic veneers, and the fracture strength of all-ceramic zirconia crowns. A lithium-disilicate
reinforced liner was applied on the surface of a zirconia core and lithium-disilicate glass-ceramic
was veneered on zirconia through heat press forming. Microtensile and crown fracture tests were
conducted in order to evaluate, respectively, the bonding strength between the zirconia cores and heat
pressed lithium-disilicate glass-ceramic veneers, and the fracture strength of bilayered zirconia
all-ceramic crowns. The role of lithium-disilicate spray-liner at the interface between zirconia
and lithium-disilicate glass-ceramic veneers was investigated through surface and cross-sectional
analyses. We confirmed that both the mean bonding strength between the zirconia ceramics and
lithium-disilicate glass-ceramic veneers and the fracture strength of the liner-treated groups were
significantly higher than those of the untreated groups, which resulted, on the one hand, from the
chemical bonding at the interface of the zirconia and lithium-disilicate liner, and, on the other,
from the existence of a microgap in the group not treated with liner.
Keywords: bilayered zirconia all-ceramic crown; lithium disilicate reinforced liner; lithium disilicate
glass-ceramic veneer; bonding strength; crown fracture strength
1. Introduction
With the increase in patients’ demands for esthetic restoration, a variety of types of ceramic
materials have been introduced to improve the esthetics of dental prostheses. Porcelain has several
advantages, such as excellent esthetics, chemical durability, and a high compressive strength. Due to
these advantages, it has been used in dental clinics as a form of porcelain jacket crown since 1887.
However, it has not been widely employed due to fracture problems during mastication. Since then,
a metal-ceramic restoration method, where a metal framework with superior mechanical properties
reinforces fragile porcelain, has been adopted for the production of esthetic prostheses. However,
since opaque prostheses resulting from both the metal framework’s interference of light and the
exposure of the margin of metal have been pointed out as the main factor in the degradation of dental
esthetics, much attention has been paid to the all-ceramics restoration method, which solely uses
ceramic materials with sufficient strength, superior biocompatibility, and penetrability of light [1,2].
Materials 2018, 11, 77; doi:10.3390/ma11010077
www.mdpi.com/journal/materials
Materials 2018, 11, 77
2 of 13
Due to their weaker strength and lower esthetics than those of metal-ceramic, several existing
all-ceramic prostheses have been restrictively used for the anterior area and not for the posterior
area or long bridge. Recent years has seen a new era opening for dental ceramics, as both zirconia
ceramics, referred to as ceramic metal, and computer-aided design (CAD)/computer-aided milling
(CAM) technology, have been introduced [3].
Zirconia refers to a zirconium (Zr) oxide that has excellent mechanical properties, even being
comparable with those of metal. Thus, Garvie et al. claimed that zirconia was a “ceramic steel”
and proposed a theoretical model around its superior mechanical properties [4]. Pure zirconia exists
as a monoclinic phase at up to 1170 ◦ C, a tetragonal phase from 1170 ◦ C to 2370 ◦ C, and a cubic
phase from 2370 ◦ C to 2680 ◦ C, the latter being the melting point as the temperature increases.
When the temperature drops after zirconia is calcined, its phase changes from cubic to tetragonal and
subsequently, to monoclinic. Among these phase transformations, a tetragonal-to-monoclinic phase
transformation shows a highly-rapid martensite-phase transformation, which cannot be suppressed
even by rapid cooling [5]. However, if a certain amount of MgO, CaO, and Y2 O3 , which are rare-earth
metal oxides, is added to pure zirconia sintered at a cubic section, and if this is then followed by
heat treatment at a section where the cubic and tetragonal phases coexist, tetragonal crystals are
precipitated within the cubic matrix and the transition to the monoclinic phase does not occur, even if
the temperature is decreased [6,7]. Thus, tetragonal zirconia polycrystals (TZP), which constitute a
metastable state after adding 3 mol % Y2 O3 , have widely been used to manufacture zirconia blocks for
CAD/CAM processing.
Zirconia has a high refractive index, which is not suitable for dental esthetics, as it can cause color
mismatch with adjacent natural teeth. Because of this drawback, when zirconia has been employed in
anterior esthetic restoration, porcelain or glass ceramic veneers have been used in the lower part of the
zirconia, in traditional all-ceramic crowns for example. Although this two-layer structure is esthetic,
there has been a problem with restoration failure due to veneers getting chipped or delaminated unless
a strong bond is guaranteed between the two materials [8,9].
There has been much research on the causes of fractures at the interface between the zirconia
core and veneer ceramics [10]. It has been shown that the fracture of the zirconia core and veneer
ceramics could result from shear stress due to the difference of the thermal expansion coefficients,
low wettability of veneer ceramics, shrinkage during firing, and the transformation of zirconia due
to heat and stress [11]. Recently, many studies have aimed to improve the mechanical and chemical
properties of zirconia surfaces in order to enhance the bonding strength between zirconia and veneer
ceramics [12–14].
In lately reported studies, lots of surface treatments have been conducted to enhance the adhesion
between zirconia and veneering ceramic [15–22]. For instance, the surface roughness of zirconia can be
increased by particle abrasion, formation of surface holes by CNC, and laser irradiation using different
types of dental lasers and energy intensities [15–18]. Also, hydrophilic functionality can be offered
by using phosphate monomer, and surface energy level can be increased by the cold atmospheric
plasma [19]. Chemical bonding can be induced by various kinds of liner treatments and selective
infiltration etching (SIE) [15,20–22]. Also, the effects of these surface treatments on bond and fracture
strengths between zirconia and veneering ceramic have been identified by various mechanical tests,
such as shear bond test, microtensile bond strength test, 3 or 4-point bending test, progressive or cyclic
load fracture test, and indentation test [23–29].
This study therefore investigated the effect of a liner treatment on the bonding strength between
zirconia cores and lithium-disilicate glass-ceramic veneers, as well as on the fracture strength of
bilayered zirconia all-ceramic crown. The null hypothesis that is proposed in the present study is
that liner treatment has no effect on either the bonding strength between zirconia cores and veneer
ceramics or the fracture strength of bilayered zirconia all-ceramic crown.
Materials 2018, 11, 77
3 of 13
2. Results
2.1. Measurement of the Bonding Strength via a Microtensile Test
Figure 1 shows the results of observations with high-resolution field emission scanning electron
Materials 2018, 11, 77
3 of 13
microscopy (HR FE-SEM), which investigated the effect of a liner treatment on the evolution of a
3Y-TZP
(yttria-stabilized
zirconia)
surface
layer.
observed
a typical
polycrystalline
structure
on
TZP (yttria-stabilized
zirconia)
surface
layer.
We We
observed
a typical
polycrystalline
structure
on the
◦
Materials
2018,
11, 77after
of 13
the
surface
of 3Y-TZP
after
maintaining
it at
1450
for2 2h,h,as
asshown
shownin
in Figure
Figure 1a.
1a. After
surface
of 3Y-TZP
maintaining
it at
1450
°C Cfor
After3applying
applying
◦ C for 90 s, the acicular crystal of the
lithium-disilicate
liner
to
3Y-TZP
and
then
incubating
it
at
950
lithium-disilicate liner to 3Y-TZP and then incubating it at 950 °C for 90 s, the acicular crystal of the
TZP (yttria-stabilized zirconia) surface layer. We observed a typical polycrystalline structure on the
lithium
disilicate
surface, acid-etched
acid-etched with
9% hydrogen
hydrogen fluoride
fluoride (HF)
(HF) aqueous
aqueous
lithium
disilicate was
was observed
observed on
on the
the surface,
with aa 9%
surface of 3Y-TZP after maintaining it at 1450 °C for 2 h, as shown in Figure 1a. After applying
solution
for
30
s,s, as
shown
in
Figure
1b.
Also,
the
structural
change
ofof
zirconia
grains
was
observed
at
solution
for
30
as
shown
in
Figure
1b.
Also,
the
structural
change
zirconia
grains
was
observed
lithium-disilicate liner to 3Y-TZP and then incubating it at 950 °C for 90 s, the acicular crystal of the
aatthin
layer
on
the
surface
of
the
specimen,
acid-etched
with
a
5%
hydrogen
fluoride
(HF)
aqueous
a thin
layer
on thewas
surface
of the
withaa9%
5%
hydrogen
fluoride
aqueous
lithium
disilicate
observed
onspecimen,
the surface,acid-etched
acid-etched with
hydrogen
fluoride
(HF)(HF)
aqueous
solution
for
aa liner
after
liner
treatment,
as
in
1c.
solution
for 30
30
min
toasremove
remove
liner layer
layer
afterthe
liner
treatment,
as shown
shown
in Figure
Figurewas
1c. observed
solution
formin
30 s,to
shown in
Figure
1b. Also,
structural
change
of
zirconia
grains
at a thin layer on the surface of the specimen, acid-etched with a 5% hydrogen fluoride (HF) aqueous
solution for 30 min to remove a liner layer after liner treatment, as shown in Figure 1c.
Figure 1. High-resolution field emission scanning electron microscopy (HR FE-SEM) images of (a) the
Figure 1. High-resolution field emission scanning electron microscopy (HR FE-SEM) images of (a) the
sintered
surface
of 3Y-TZP,field
andemission
the acid-etched
surfaces
with a (HR
(b) 9%
hydrogen
Figure
1. High-resolution
scanning electron
microscopy
FE-SEM)
imagesfluoride
of (a) the (HF)
sintered surface of 3Y-TZP, and the acid-etched surfaces with a (b) 9% hydrogen fluoride (HF) solution
sintered
surface
and thefluoride
acid-etched
with
9%after
hydrogen
fluoride
(HF)
solution
for 30
s andof(c)3Y-TZP,
5% hydrogen
(HF)surfaces
solution
for a30(b)min
a spray
liner treatment
for 30 s and (c) 5% hydrogen fluoride (HF) solution for 30 min after a spray liner treatment respectively.
solution for 30 s and (c) 5% hydrogen fluoride (HF) solution for 30 min after a spray liner treatment
respectively.
respectively.
Figure 2 shows the HR FE-SEM-obtained cross-sectional image of the bonding interface after a
Figure 2 shows the HR FE-SEM-obtained cross-sectional image of the bonding interface after a
Figure 2on
shows
the HR
image
of the
interface
afterfor
a 30 s
liner treatment
3Y-TZP.
TheFE-SEM-obtained
specimen was cross-sectional
acid-etched with
a 9%
HFbonding
aqueous
solution
liner treatment on 3Y-TZP. The specimen was acid-etched with a 9% HF aqueous solution for 30 s
liner
treatment
on
3Y-TZP.
The
specimen
was
acid-etched
with
a
9%
HF
aqueous
solution
for
30
s that
after being cut with a low-speed diamond cutter to observe the bonding interface. We identified
after after
being
cut with
a low-speed
diamond
cutter
to observe
observethe
thebonding
bonding
interface.
We
identified
that
being
cut
with
a
low-speed
diamond
cutter
to
interface.
We
identified
that
bonding occurred with the formation of a reaction layer on 3Y-TZP’s surface, as shown in Figure 2a.
bonding
occurred
with
thethe
formation
layeron
on3Y-TZP’s
3Y-TZP’s
surface,
as shown
in Figure
bonding
occurred
with
formationofofaareaction
reaction layer
surface,
as shown
in Figure
2a. 2a.
Furthermore, the energy-dispersive X-ray spectroscopy (EDS) analysis results for the reaction layer,
Furthermore,
the the
energy-dispersive
(EDS)analysis
analysis
results
reaction
Furthermore,
energy-dispersiveX-ray
X-rayspectroscopy
spectroscopy (EDS)
results
for for
the the
reaction
layer,layer,
as shown in Figure 2b, revealed that there was no significant change in the reaction layer adjacent to
as shown
in Figure
2b,2b,
revealed
that
significantchange
change
reaction
adjacent
to
as shown
in Figure
revealed
thatthere
therewas
was no significant
in in
thethe
reaction
layerlayer
adjacent
to
the liner,
whereas
a reduction
of the
Sithe
peak
and elevation
of the
Zr
peak
werewere
identified
in the
reaction
the
liner,
whereas
a
reduction
of
Si
peak
and
elevation
of
the
Zr
peak
identified
in
the
the liner, whereas a reduction of the Si peak and elevation of the Zr peak were identified in the
layerreaction
that was adjacent
to adjacent
the zirconia.
thezirconia.
zirconia.
reaction layerlayer
thatthat
waswas
adjacent
totothe
Figure 2. (a) HR FE-SEM cross-sectional image and (b) energy-dispersive X-ray spectroscopy (EDS)
Figure 2. (a) HR FE-SEM cross-sectional image and (b) energy-dispersive X-ray spectroscopy (EDS)
line analysis of the bonding interface after liner treatment on 3Y-TZP.
line analysis of the bonding interface after liner treatment on 3Y-TZP.
Figure 2. (a) HR FE-SEM cross-sectional image and (b) energy-dispersive X-ray spectroscopy (EDS)
Figure 3 shows the results of the X-ray analysis on the surface of the liner-treated 3Y-TZP. In
line analysis of the bonding interface after liner treatment on 3Y-TZP.
addition to α-lithium disilicate (α-Li2Si2O5), zirconia (ZrO2), and silica (SiO2) peaks, we observed
peaks for lithium metasilicate (Li2SiO3) and zirconium silicate (ZrSiO4).
Figure 3 shows the results of the X-ray analysis on the surface of the liner-treated 3Y-TZP. In
addition to α-lithium disilicate (α-Li2Si2O5), zirconia (ZrO2), and silica (SiO2) peaks, we observed
peaks for lithium metasilicate (Li2SiO3) and zirconium silicate (ZrSiO4).
Materials 2018, 11, 77
4 of 13
Figure 3 shows the results of the X-ray analysis on the surface of the liner-treated 3Y-TZP.
In addition to α-lithium disilicate (α-Li2 Si2 O5 ), zirconia (ZrO2 ), and silica (SiO2 ) peaks, we observed
Materials
2018,
11, 77 metasilicate (Li SiO ) and zirconium silicate (ZrSiO ).
4 of 13
peaks for
lithium
2
3
4
3. The
X-ray diffraction
(XRD)on
pattern
on theof
surface
of liner-treated
Figure 3.Figure
The X-ray
diffraction
(XRD) pattern
the surface
liner-treated
3Y-TZP. 3Y-TZP.
Figure
shows the
tensile bonding
bonding strength
molding with
with Amber
Amber LiSi-POZ
Figure 44 shows
the tensile
strength after
after heat
heat press
press molding
LiSi-POZ of
of
both
The mean
mean tensile
tensile bonding
bonding strength
strength of
both the
the untreated
untreated and
and liner-treated
liner-treated 3Y-TZP.
3Y-TZP. The
of the
the untreated
untreated and
and
treated
18.83 MPa
MPa and
and 44.20
44.20 MPa,
MPa, respectively;
respectively; aa statistically
statistically significant
significant high
high bonding
bonding
treated groups
groups was
was 18.83
strength
was
revealed
in
the
liner-treated
group
(p
<
0.01).
strength was revealed in the liner-treated group (p < 0.01).
Figure 4.
4. Microtensile
Microtensile bonding
bonding strengths
strengths after
after heat
heat press
press molding
molding with
with Amber
Amber LiSi-POZ
LiSi-POZ of
of both
both the
the
Figure
untreated and liner-treated 3Y-TZP.
untreated and liner-treated 3Y-TZP.
2.2. Measurement
Measurement of
of the
the Fracture
Fracture Strength
Strength via
2.2.
via aa Compression
Compression Test
Test
Figure 55 shows
shows the
the Weibull
Weibull plots
and liner-treated
liner-treated
Figure
plots of
of the
the fracture
fracture strengths
strengths for
for the
the untreated
untreated and
bilayered
zirconia
all-ceramic
crowns
after
a
crown
fracture
test,
and
Table
1
shows
the
data
from
bilayered zirconia all-ceramic crowns after a crown fracture test, and Table 1 shows the data
from
a
a
Weibull
analysis.
The
Weibull
distribution
showed
a
matched
tendency
with
the
single
mode
Weibull analysis. The Weibull distribution showed a matched tendency with the single mode
(r22 >> 0.86).
0.86). The
The Weibull
Weibull coefficient
than that
that of
of the
the untreated
(r
coefficient of
of the
the liner-treated
liner-treated group
group was
was higher
higher than
untreated
group. Additionally, the liner-treated group’s mean fracture strength was, statistically speaking,
significantly higher than that of the untreated group (p < 0.01).
Materials 2018, 11, 77
5 of 13
group. Additionally, the liner-treated group’s mean fracture strength was, statistically speaking,
5 of 13
significantly higher than that of the untreated group (p < 0.01).
Materials 2018, 11, 77
Figure 5.
The Weibull
Weibull plots
plots of
of the
the fracture
fracture strengths
for the
Figure
5. The
strengths for
the untreated
untreated and
and liner-treated
liner-treated bilayered
bilayered
zirconia all-ceramic
zirconia
all-ceramic crowns.
crowns.
data
of the
strengths
for the for
untreated
and liner-treated
bilayered
Table
Table 1.1.The
TheWeibull-analysis
Weibull-analysis
data
of fracture
the fracture
strengths
the untreated
and liner-treated
zirconia
all-ceramic
crowns.
bilayered zirconia all-ceramic crowns.
Group
Group
Parameter
Parameter
m m
σo σo
r2 r 2
σf(avg) ± SD
σf(avg) ± SD
N
N
Untreated
Untreated
Liner-Treated
Liner-Treated
2.87
2.87
2.45
2.45
0.86
0.86
2.18 ± 0.85
2.18 ± 0.85
15
4.684.68
3.773.77
0.910.91
± 0.80
3.453.45
± 0.80
15
15
15
m = Weibull modulus; σ0 = characteristic strength in kN; r2 = Weibull-distribution regression coefficient squared;
2
m
= Weibull
modulus;
σ0 in
= N;
characteristic
σf(avg)
= mean fracture
strength
N = number ofstrength
samples. in kN; r = Weibull-distribution regression
coefficient squared; σf(avg) = mean fracture strength in N; N = number of samples.
Figure 6a shows three locations for the HR FE-SEM observation on the fractured bilayered zirconia
Figure 6a shows three locations for the HR FE-SEM observation on the fractured bilayered
all-ceramic crown (A: the veneer’s fracture surface, B: the interface between veneer and core, and C:
zirconia all-ceramic crown (A: the veneer’s fracture surface, B: the interface between veneer and core,
the core surface) after the crown fracture test. Figure 6b,c shows HR FE-SEM images of the fractured
and C: the core surface) after the crown fracture test. Figure 6b,c shows HR FE-SEM images of the
surfaces of untreated and liner-treated crowns after the crown fracture test. We observed a typical
fractured surfaces of untreated and liner-treated crowns after the crown fracture test. We observed a
acicular crystal of lithium disilicate on the veneer fracture surface (A1,A2). In the untreated group,
typical acicular crystal of lithium disilicate on the veneer fracture surface (A1,A2). In the untreated
at the interface between the veneer and the 3Y-TZP core, we identified that a small reactive layer was
group, at the interface between the veneer and the 3Y-TZP core, we identified that a small reactive
generated on the 3Y-TZP surface (B1), whereas in the liner-treated group, a thick reactive layer formed
layer was generated on the 3Y-TZP surface (B1), whereas in the liner-treated group, a thick reactive
on the 3Y-TZP surface (B2). The untreated group (C1) showed a similar pattern to that of the sintered
layer formed on the 3Y-TZP surface (B2).
The untreated group (C1) showed a similar pattern to that
surface on the core’s surface, at 1450 ◦ C after milling the block. In the liner-treated group (C2), on the
of the sintered surface on the core’s surface, at 1450 °C after milling the block. In the liner-treated
other hand, the reactive layer with the liner completely covered the 3Y-TZP surface.
group (C2), on the other hand, the reactive layer with the liner completely covered the 3Y-TZP
Figure 7 shows cross-sectional HR FE-SEM images of the bond interface of the untreated and
surface.
liner-treated bilayered zirconia all-ceramic crowns before and after acid etching. It was difficult to
Figure 7 shows cross-sectional HR FE-SEM images of the bond interface of the untreated and
detect the difference at the bond interface of the untreated and liner-treated all-ceramic zirconia crowns
liner-treated bilayered zirconia all-ceramic crowns before and after acid etching. It was difficult to
before acid etching, as shown in Figure 7a,b. However, a microgap was observed at the bond interface
detect the difference at the bond interface of the untreated and liner-treated all-ceramic zirconia
in the untreated crown, as shown in Figure 7b, but we could confirm a well-combined pattern in the
crowns before acid etching, as shown in Figure 7a,b. However, a microgap was observed at the bond
liner-treated crown after acid etching, as shown in Figure 7d.
interface in the untreated crown, as shown in Figure 7b, but we could confirm a well-combined
pattern in the liner-treated crown after acid etching, as shown in Figure 7d.
Materials 2018, 11, 77
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2018,11,
11,77
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Materials
6 of 13
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13
66of
Figure
6.
(a)
Photograph
presenting
three
locations
of
the
fractured
bilayered
zirconia
Figure 6.
6. (a)
(a) Photograph
Photograph presenting
presenting three
three locations
locations of
of the
the fractured
fractured bilayered
bilayered zirconia
zirconia all-ceramic
all-ceramic
Figure
all-ceramic
crown
for
HR
FE-SEM
observation;
and
HR
FE-SEM
images
for
(b)
the
fractured
surfaces
of
the
crown
for
HR
FE-SEM
observation;
and
HR
FE-SEM
images
for
(b)
the
fractured
surfaces
of the
the
crown for HR FE-SEM observation; and HR FE-SEM images for (b) the fractured surfaces of
untreated
and
(c)
liner-treated
bilayered
zirconia
all-ceramic
crown
s;s; (A:
the
veneer’s
fracture surface,
surface,
untreated
and
(c)
liner-treated
bilayered
zirconia
all-ceramic
crown
(A:
the
veneer’s
fracture
untreated and (c) liner-treated bilayered zirconia all-ceramic crown s; (A: the veneer’s fracture surface,
B:
the
interface
between
veneer
and
core,
B: the
theinterface
interfacebetween
between veneer
veneerand
and core,
core,and
andC:
C:the
thecore
coresurface).
surface).
B:
and
C:
the
core
surface).
Figure 7.
7. Cross-sectional
Cross-sectional HR
HR FE-SEM
FE-SEM images
images of
of the
the bond
bond interface
interface with
with and
and without
without liner
liner treatment
treatment
Figure
Figure 7. Cross-sectional HR FE-SEM images of the bond interface with and without liner treatment
showing
the
untreated
bilayered
zirconia
all-ceramic
crown
(a)
before
and
(b)
after
acid
etching
with
showing the untreated bilayered zirconia all-ceramic crown (a) before and (b) after acid etching with
showing the untreated bilayered zirconia all-ceramic crown (a) before and (b) after acid etching with
9%HF
HFsolution
solutionfor
for30
30s;s;and
andthe
theliner-treated
liner-treatedbilayered
bilayeredzirconia
zirconiaall-ceramic
all-ceramiccrown
crown(c)
(c)before
beforeand
and(d)
(d)
aa9%
a 9% HF solution for 30 s; and the liner-treated bilayered zirconia all-ceramic crown (c) before and
after
acid
etching
with
a
9%
HF
solution
for
30
s.
after
acidacid
etching
withwith
a 9%
HF HF
solution
for for
30 s.30 s.
(d) after
etching
a 9%
solution
3. Discussion
Discussion
3.
3. Discussion
We conducted
conducted this
this study
study in
in order
order to
to investigate
investigate what
what the
the effect
effect of
of aa lithium-disilicate
lithium-disilicate liner
liner
We
We conducted
this
study in
order core
to investigate
what
the
effect strength
of a lithium-disilicate
liner
treatment
applied
to
a
3Y-TZP
zirconia
would
be
on
the
bonding
of
lithium-disilicate
treatment applied to a 3Y-TZP zirconia core would be on the bonding strength of lithium-disilicate
treatment
applied
to a and
3Y-TZP
zirconia
core
wouldof
bean
onbilayered
the bonding
strength
of lithium-disilicate
glass-ceramic
veneers
on the
the
fracture
strength
zirconia
all-ceramic
crown. ItIt was
was
glass-ceramic
veneers
and on
fracture
strength
of an
bilayered zirconia
all-ceramic
crown.
glass-ceramic
veneers
and
on
the
fracture
strength
of
an
bilayered
zirconia
all-ceramic
crown. Itatwas
confirmed
that
the
micro-gap
dissipated
after
the
linear
treatment
due
to
the
chemical
reaction
the
confirmed that the micro-gap dissipated after the linear treatment due to the chemical reaction at the
confirmed
that
the
micro-gap
dissipated
after
the
linear
treatment
due
to
the
chemical
reaction
at
the
interfacebetween
betweenzirconia
zirconiacore
coreand
andveneering
veneeringceramic
ceramicduring
duringthe
theheat
heatpress
pressforming
formingof
ofthe
theveneering
veneering
interface
ceramics,
as
shown
in
Figures
2a
and
7d.
Thus,
the
microtensile
test
was
conducted
to
evaluate
the
ceramics, as shown in Figures 2a and 7d. Thus, the microtensile test was conducted to evaluate the
bond
strength
between
the
zirconia
core
and
the
veneering
ceramic.
Crown
fracture
test
was
also
bond strength between the zirconia core and the veneering ceramic. Crown fracture test was also
Materials 2018, 11, 77
7 of 13
interface between zirconia core and veneering ceramic during the heat press forming of the veneering
ceramics, as shown in Figures 2a and 7d. Thus, the microtensile test was conducted to evaluate
the bond strength between the zirconia core and the veneering ceramic. Crown fracture test was
also conducted to identify the effect of the microgap on the fracture strength of veneering ceramic
when the load is applied on the bilayered zirconia all-ceramic crown consisting of zirconia core and
upper veneering ceramic. The results of Figure 4 and Table 1 showed that both the bonding and
fracture strengths of the liner-treated group were significantly higher than them of the untreated group,
from which we concluded that the present study’s null hypothesis could be rejected.
The strength of the material is an important factor in the chipping of veneer ceramics in
zirconia all-ceramic crowns that were made of a zirconia core and upper veneer ceramics. However,
both bonding and a matching thermal-expansion coefficient between the core and the veneer hold the
highest importance, following the rationale that tensile stress occurring inside veneer ceramic may
have a strong influence on chipping. Besides, structural improvements—such as a thickening of the
vulnerable areas in veneers where high-tensile stress occurs, a structural design where the core can act
as the support, and the prevention of pore-formation inside the sintered body and microgap-formation
at the interfaces—are required in order to prevent the formation, via tensile stress, of chipping fractures
in veneers [30].
The mismatch of the thermal-expansion coefficient between the core and veneer induces residual
tensile stress, which can lead to chipping creation [31]. Since the criteria defining a range for the
thermal-expansion coefficient—a range that would be thermally suitable for zirconia all-ceramic
restoration—are yet to be clarified, the thermal-expansion coefficient for veneers has been engineered,
in metal-ceramic restoration, for instance, to have a slightly smaller value than that of the zirconia core.
This aims to induce residual compressive stress on the ceramic-veneer surface. Nonetheless, this has
been an issue that generates tensile stress on the surface of zirconia cores [32]. Several researchers have
suggested that since residual tensile stress that is caused by the mismatch of the thermal-expansion
coefficients may have harmful effects on veneers as well as cores, the thermal-expansion coefficients
should be matched for ceramic restoration [33,34].
Since zirconia is chemically stable at a high temperature, its bonding strength with veneer ceramics
is low [14]. To overcome this drawback, there have been a number of studies on how to change the
structure of zirconia’s surface layer so as to induce mechanical bonding [13,14], and on how liner
treatment at the interface induces bonding [12]. The alumina-powder blasting treatment is one of the
most frequently used surface-treatment methods for obtaining a mechanical retaining force by changing
the surface’s roughness. However, blasting treatment was unable to achieve a high bonding strength
due to its limited ability to expand the surface area of polycrystalline zirconia [13]. Since zirconia
exhibits excellent stability at a high temperature, it shows chemically inert characteristics for a range
of porcelain plastic temperatures. However, when atoms such as Si, Na, K, or Mg are present on the
surface, slips, segmentations, and rearrangements of grain boundaries occur. Using this phenomenon,
a selective-infiltration etching technique was proposed in order to obtain a microporous structure on
the surface [12]. In the present study, we observed a structural change in a thin surface layer of the
zirconia in Figure 1c when a lithium-disilicate liner was applied to 3Y-TZP, which was then maintained
at 950 ◦ C for 90 s and immersed in a 5% HF aqueous solution for 30 min to remove a layer of liner.
We could attribute this structural change in zirconia at the surface layer to the influence of Li and
Si, which were included in the liner components, as proposed by Aboushelib et al. [12]. A reaction
layer at the interface was generated due to a thermochemical reaction between the zirconia and liner,
in which zirconium silicate (ZrSiO4 ) was generated in a layer adjacent to the zirconia, while lithium
metasilicate (Li2 SiO3 ) was generated in a layer adjacent to the lithium-disilicate liner, as confirmed in
Figures 2 and 3.
Additionally, the tensile bonding strength of the group treated with the lithium-disilicate
reinforced spray-type liner was higher than that of the untreated group, and this was because of
the formation of an acicular structure of lithium disilicate in the liner layer during the heat press
Materials 2018, 11, 77
8 of 13
forming of the veneer ceramics, as confirmed in Figure 1b. Additionally, strong bonding resulted from
the formation of zirconium silicate through the reaction of zirconia and silicon dioxide (the liner’s
main component)
Materials
2018, 11, 77 in the surface layer adjacent to the zirconia, whereas a microgap was observed at
8 ofthe
13
interface between the 3Y-TZP zirconia core and veneer in the group that was not treated with liner,
as confirmed
Figure 7.
After theinfracture
test of the all-ceramic zirconia crown, an interfacial fracture was revealed
Afterthe
theveneer
fracture
of the core
all-ceramic
crown,
an interfacial
wasthe
revealed
between
andtest
zirconia
for the zirconia
untreated
all-ceramic
zirconia fracture
crown. For
linerbetween
the
veneer
and
zirconia
core
for
the
untreated
all-ceramic
zirconia
crown.
For
the
liner-treated
treated all-ceramic zirconia crown, a cohesive fracture was revealed in the reaction layer between the
all-ceramic
zirconia
crown,
a cohesive
fracturebetween
was revealed
in the
between
liner
liner
and core,
resulting
from
strong bonding
the core
andreaction
veneer layer
materials.
The the
fracture
and core,ofresulting
from
strong
between
thefor
core
veneer materials.
The for
fracture
strength
strength
the single
crown
wasbonding
significantly
higher
theand
liner-treated
crown than
the untreated
of the single
crown
was significantly
for was
the liner-treated
crown
than
for the untreated
crown,
crown,
as was
the Weibull
coefficient,higher
and this
also confirmed
by the
figuration
of the fracture.
as was
coefficient,
andthat,
thiswhen
was also
confirmed
by the
figuration
the fracture.
This can
This
canthe
beWeibull
explained
by the fact
external
force was
applied
to theofcrown,
like Figure
8a,
be explained
by the
fact that,
when external
force was
applied
to to
thethe
crown,
Figure 8a, for
for
the untreated
group,
the resistance
to fractures
dropped
due
stresslike
concentration
at the
untreated which
group, resulted
the resistance
fractures
dropped
to the stress
concentration
at theveneer,
interface,
interface,
from tothe
generation
of a due
microgap
between
the core and
as
which resulted
from8b.
theFor
generation
of a microgap
between
the core
and veneer,
as confirmed
in
confirmed
in Figure
the liner-treated
group, the
resistance
to fractures
rose due
to the stress
Figure
8b.
For
the
liner-treated
group,
the
resistance
to
fractures
rose
due
to
the
stress
relaxation
at
the
relaxation at the interface, which resulted from both the elimination of defects in the veneer through
interface,
which
resulted
of the
defects
the veneerthe
through
of the
the
binding
of the
veneerfrom
and both
liner,the
andelimination
the fact that
coreinsupported
veneer,the
asbinding
confirmed
in
veneer 8c.
and liner, and the fact that the core supported the veneer, as confirmed in Figure 8c.
Figure
Figure 8.
8. (a)
(a)An
Anillustration
illustrationshowing
showing
direction
stress
at the
edge
of bilayered
zirconia
Figure
thethe
direction
of of
thethe
stress
at the
edge
of bilayered
zirconia
allall-ceramic
crown
when
external
is applied
it; HR
andFE-SEM
HR FE-SEM
images
and figures
the
ceramic
crown
when
external
forceforce
is applied
to it; to
and
images
and figures
of the of
stress
stress distribution
the interface
between
the core
veneer
untreatedand
and(c)
(c) liner-treated
liner-treated
distribution
at theatinterface
between
the core
andand
veneer
forfor
(b)(b)untreated
bilayered zirconia all-ceramic crowns.
bilayered
4. Materials
Materials and
and Methods
Methods
4.1. Measurement
Measurement of
of Bonding
Bonding Strength
Strength
4.1.
4.1.1. Preparation
Preparation of
of Specimens
Specimens
4.1.1.
To prepare
of the
zirconia-sintered
body,body,
a calcined
3Y-TZP3Y-TZP
disc block
was
prepared
To
preparethe
thespecimens
specimens
of the
zirconia-sintered
a calcined
disc
block
was
for
CAD/CAM
processing
(Zirtooth™
O98FGJ1701,
Hass,
Gangneung,
Korea):
it
was
sintered
at
prepared for CAD/CAM processing (Zirtooth™ O98FGJ1701, Hass, Gangneung, Korea): it was
◦ C for 2 h and then cut so as to measure 12 mm × 12 mm× 7 mm. 50 µm alumina abrasives
1450
sintered at 1450 °C for 2 h and then cut so as to measure 12 mm × 12 mm× 7 mm. 50 μm alumina
(Hi-aluminas,
Shofu, Japan)
were
sprayed
with
a pressure
3 atm from
mm in order
abrasives
(Hi-aluminas,
Shofu,
Japan)
were
sprayed
with of
a pressure
of a3 distance
atm fromofa10
distance
of 10
to
increase
surface
roughness
in
the
coupling
portion.
The
specimens
were
put
into
an
electric
furnace
mm in order to increase surface roughness in the coupling portion. The specimens were put into an
◦ C/min to 1000 ◦ C, and then maintained for
and thefurnace
temperature
was
increased atwas
a heating
rateat
ofa50
electric
and the
temperature
increased
heating
rate of 50 °C/min to 1000 °C, and then
10
min
in
order
to
recover
a
phase-transition
layer
generated
on
the
during
maintained for 10 min in order to recover a phase-transition layersurface
generated
on the
thespray-treatment
surface during
process.
Finally, the specimens
were then
subjected to
furnace
the
spray-treatment
process. Finally,
the specimens
were
then cooling.
subjected to furnace cooling.
The prepared specimens were randomly divided into the untreated and liner-treated group. For
the liner-treated group, a lithium-disilicate reinforced spray-type liner (Hass, Gangneung, Korea),
composed of SiO2 < 82 wt %, Li2O > 10 wt %, P2O5 < 7 wt %, and other oxides and colorant < 6 wt %,
was coated once on the surface of the 3Y-TZP, and the temperature was increased at a heating rate of
60 °C/min to 940 °C and then maintained for 90 s. Following this, a wax pattern that was the same
Materials 2018, 11, 77
9 of 13
The prepared specimens were randomly divided into the untreated and liner-treated group.
For the liner-treated group, a lithium-disilicate reinforced spray-type liner (Hass, Gangneung, Korea),
composed of SiO2 < 82 wt %, Li2 O > 10 wt %, P2 O5 < 7 wt %, and other oxides and colorant < 6 wt %,
was coated once on the surface of the 3Y-TZP, and the temperature was increased at a heating rate
Materials 2018, 11, 77
9 of 13
of 60 ◦ C/min to 940 ◦ C and then maintained for 90 s. Following this, a wax pattern that was the
samesize
sizeasasthe
the
zirconia-sintered
body
attached
tothe
both
the untreated
the liner-treated
zirconia-sintered
body
was was
attached
to both
untreated
and the and
liner-treated
group, andgroup,
the investment
investment material
Vest
HS,
6-51198-59,
Seichong,
Seoul,
Korea)
was used
to make
a
and the
material(Prime
(Prime
Vest
HS,
6-51198-59,
Seichong,
Seoul,
Korea)
was used
to make
a
◦
◦
mold.
After
burning
out
the
wax
at
850
°C
for
50
min,
we
conducted
heat
pressing
at
925
°C
using
mold. After burning out the wax at 850 C for 50 min, we conducted heat pressing at 925 C using
lithium-disilicate
glass-ceramic
ingot
(AmberLiSi-POZ,
LiSi-POZ, Hass,
Hass, Gangneung,
as as
shown
in Table
lithium-disilicate
glass-ceramic
ingot
(Amber
Gangneung,Korea),
Korea),
shown
in Table 2.
2.
50 µm glass beads (Rolloblast, Renfert GmbH, Hilzingen, Germany) were blasted at a pressure of
50 μm glass beads (Rolloblast, Renfert GmbH, Hilzingen, Germany) were blasted at a pressure
2 atm to remove the investment material that was attached to the specimen’s surface. Then, ultrasonic
of 2 atm to remove the investment material that was attached to the specimen’s surface. Then,
cleaning was conducted in deionized water for 5 min. The specimens were then fixed to a low-speed
ultrasonic cleaning was conducted in deionized water for 5 min. The specimens were then fixed to a
diamond
cutterdiamond
and cutcutter
into 1.1
× 1.1
Four
of these
samples
were were
takentaken
from the
low-speed
andmm
cut into
1.1mm
mm sizes.
× 1.1 mm
sizes.
Four ofcut
these
cut samples
center
of each
specimen.
9 shows
schematic
drawing drawing
to indicate
dimensions
of the of
specimens
from
the center
of eachFigure
specimen.
Figure
9 shows schematic
to indicate
dimensions
the
prepared
for
the
microtensile
bond
testing.
specimens prepared for the microtensile bond testing.
Table
Firingschedules
schedules of
material.
Table
2. 2.Firing
of the
theveneering
veneering
material.
Veneering
Veneering
MaterialsMaterials
ST (◦ C)
Amber LiSi-POZ
650
Amber LiSi-POZ
ST
DT
TRI
DT (min)
TRI (◦ C/min)
(°C)
(min)
(°C/min)
60 60
650 1
1
FT
(°C)
925
925
FT (◦ C)
V1
(°C)
650
650
V1 (◦ C)
V2 ◦
HT
V2 ( C)
HT (min)
(°C)
(min)
925925
20 20
ST: starting
temperature;
DT: drying
time; FT:
finalFT:
temperature;
TRI: temperature
rate increase;
V1: temperature
of
ST: starting
temperature;
DT: drying
time;
final temperature;
TRI: temperature
rate increase;
V1:
vacuum on; V2: temperature of vacuum off; HT: holding time.
temperature of vacuum on; V2: temperature of vacuum off; HT: holding time.
Figure
9. Schematicdrawing
drawing of
coupled
block
of 3Y-TZP
zirconiazirconia
and LiSi-POZ
disilicate glassFigure
9. Schematic
of(a)
(a)the
the
coupled
block
of 3Y-TZP
and LiSi-POZ
disilicate
ceramic
after
heat
pressing
and
(b)
the
final
specimen
prepared
for
microtensile
bond
testing.
glass-ceramic after heat pressing and (b) the final specimen prepared for microtensile bond testing.
4.1.2. Microtensile Test
4.1.2. Microtensile Test
A metal holder was attached to both sides of the specimen for microtensile bond testing. The
A metal specimens
holder was
sidestesting
of the
specimen
microtensile
bond
testing.
prepared
wereattached
mounted to
to aboth
universal
machine
(4201,for
Instron
Co., Norwood,
MA,
The prepared
mounted
a universal
testing
(4201,
Co.,aNorwood,
USA), andspecimens
tensile forcewere
was applied
at atocrosshead
speed
of 0.5 machine
mm/min in
orderInstron
to measure
load
MA, at
USA),
and tensile
was
applied
at tensile-bond
a crossheadstrength
speed of
0.5calculated
mm/min
order to
measure
the moment
of an force
interface
fracture.
The
was
byin
dividing
a load
a load
momentin
ofthe
an coupling
interfaceportion.
fracture.
The tensile-bond
strength
calculated
bylinerdividing
byata the
cross-section
Twelve
specimens from
each of was
the untreated
and
treated
groups were used
for the
microtensile
bond
testing.specimens from each of the untreated and
a load
by a cross-section
in the
coupling
portion.
Twelve
liner-treated groups were used for the microtensile bond testing.
4.2. Fracture Test of All-Ceramic Zirconia Crown
4.2. Fracture Test of All-Ceramic Zirconia Crown
4.2.1. Preparation of Specimens
4.2.1. Preparation
Specimens
A model ofofthe
mandibular first molars of permanent teeth was scanned. Thirty metal abutments
were prepared following a customized cutback design. After designing the zirconia coping with a 0.5
A model of the mandibular first molars of permanent teeth was scanned. Thirty metal abutments
mm thickness, a 3Y-TZP disc block (Zirtooth™ O98FGJ1701, Hass, Gangneung, Korea) was processed
were prepared following a customized cutback design. After designing the zirconia coping with
with the milling machine M1 (Zirkonzahn, South Tyrol, Italy) in order to prepare a total of 30 copings,
a 0.5which
mm thickness,
3Y-TZP
disc block
(Zirtooth™
Hass, Gangneung,
Korea) was
were then a
put
into a zirconia
sintering
furnaceO98FGJ1701,
(HTC0116, Nabertherm
GmbH, Lilienthal,
processed
withwhere
the milling
machinewas
M1increased
(Zirkonzahn,
South
Tyrol,
Italy) in
prepare
a total
Germany)
the temperature
at a heating
rate
of 3 °C/min
uporder
to 1450to°C,
and then
maintained for two hours.
Materials 2018, 11, 77
10 of 13
of 30 copings, which were then put into a zirconia sintering furnace (HTC0116, Nabertherm GmbH,
Lilienthal, Germany) where the temperature was increased at a heating rate of 3 ◦ C/min up to 1450 ◦ C,
and then maintained for two hours.
All of the
copings were treated by blasting with 50 µm alumina abrasives (Hi-aluminas,
Materials
2018,zirconia
11, 77
10 of 13
Shofu, Japan) at a pressure of 3 atm from a distance 10 mm in order to increase surface roughness in
All ofportion.
the zirconia
copings were
by into
blasting
50 μm and
alumina
abrasives (Hi-aluminas,
the coupling
Specimens
weretreated
divided
the with
untreated
liner-treated
groups, each with
Shofu,
Japan)
at
a
pressure
of
3
atm
from
a
distance
10
mm
in
order
to
increase
surface roughness
in
15 randomly allocated specimens: the liner-treated group was obtained by increasing
the temperature
the
coupling
portion.
Specimens
were
divided
into
the
untreated
and
liner-treated
groups,
each
with
to 940 ◦ C and maintaining it there for 90 s after applying one coating of spray-type liner.
15 randomly allocated specimens: the liner-treated group was obtained by increasing the temperature
A crown wax pattern designed and prepared with CAD/CAM was bound to the copings of both
to 940 °C and maintaining it there for 90 s after applying one coating of spray-type liner.
the untreated
and liner-treated groups, and their molds were made using investment (prime vest
A crown wax pattern designed and prepared with CAD/CAM was bound to the copings of both
HS, the
BK untreated
Giulini, Ludwigshafen,
withwere
water
at ausing
ratioinvestment
of 0.25 and
thenvest
leftHS,
at room
and liner-treated Germany)
groups, and mixed
their molds
made
(prime
◦ C for 50 min, after which they were coupled to
temperature
for
30
min.
The
molds
were
heated
at
850
BK Giulini, Ludwigshafen, Germany) mixed with water at a ratio of 0.25 and then left at room
a disilicate
glass-ceramic
(Amber
LiSi-POZ,
Korea)
into
the electric
temperature
for 30 min.ingot
The molds
were
heated at Hass,
850 °CGangneung,
for 50 min, after
whichand
theyput
were
coupled
to a(Programat
disilicate glass-ceramic
(Amber
LiSi-POZ,
Hass,
Gangneung, Korea)
and put
into
the
furnace
EP3000/G2,ingot
Ivoclar
Vivadent,
Schaan,
Liechtenstein)
where they
were
maintained
◦ C for
electric
furnace
(Programat
EP3000/G2,
Ivoclar
Vivadent,
Schaan, Liechtenstein) where they were
at 915
15 min,
before being
subjected
to heat
press molding.
maintained
at 915was
°C for
15 min, before
being was
subjected
heat
pressthe
molding.
After molding
completed,
the mold
takentoout
from
electric furnace and then cooled
After molding was completed, the mold was taken out from the electric furnace and then cooled
until it became a room temperature. Blasting with 50 µm glass beads was performed at a pressure
until it became a room temperature. Blasting with 50 μm glass beads was performed at a pressure of
of 2 atm to remove the investment material attached to the crown’s surface. In order to make it
2 atm to remove the investment material attached to the crown’s surface. In order to make it uniform,
uniform,
the surface was then sequentially polished using Carborundum Wheel (GC#22, Sunil Ceramic
the surface was then sequentially polished using Carborundum Wheel (GC#22, Sunil Ceramic IND
INDCo.,
Co.,Gyunggido,
Gyunggido,Korea),
Korea),
Carborundum
Point
(GC#20,
Sunil
Ceramic
Co., Gyunggido,
Korea),
Carborundum
Point
(GC#20,
Sunil
Ceramic
IND IND
Co., Gyunggido,
Korea),
®
®
Silicone
Wheel
(YoudentTM
Classic,
China),
and
Rubber
Wheel
(Dedeco
Classic
Silicone
Wheel
(YoudentTM
Classic,Youdent,
Youdent, Shanghai,
Shanghai, China),
and
Rubber
Wheel
(Dedeco
Classic
No. No.
5001,
Dedeco
LongEddy,
Eddy,NY,
NY,USA).
USA).
finishing,
we used
Kohinoor
L
5001,
DedecoInternational
International Inc.,
Inc., Long
ForFor
finishing,
we used
Kohinoor
L
(Diamond
Polishing
Paste
No.516-0001,
516-0001, Renfert
Renfert GmbH,
Germany).
Figure
10 shows
the the
(Diamond
Polishing
Paste
No.
GmbH,Hilzingen,
Hilzingen,
Germany).
Figure
10 shows
optical
images
of the
specimenatateach
each stage
stage of
process
prior
to the
test for
optical
images
of the
specimen
ofthe
thepreparation
preparation
process
prior
to fracture
the fracture
test for
the
all-ceramic-zirconia-crown.
the all-ceramic-zirconia-crown.
Figure
10. Photograph
of of
thethe
specimen
processprior
priortotothe
thefracture
fracture test
Figure
10. Photograph
specimenatateach
eachstage
stageof
ofthe
the preparation
preparation process
testall-ceramic
for the all-ceramic
zirconia
(a) custom-milled
abutment,
milled
zirconia
coping,(c)
(c)milled
for the
zirconia
crown;crown;
(a) custom-milled
abutment,
(b) (b)
milled
zirconia
coping,
crownand
pattern,
and (d) pressed
Lisi-POZ
crown.
wax milled
crownwax
pattern,
(d) pressed
Amber Amber
Lisi-POZ
crown.
4.2.2. Cementation
4.2.2. Cementation
The metal abutments and the inside of the prepared crown were treated with 32% phosphoric
The metal abutments and the inside of the prepared crown were treated with 32% phosphoric
acid gel (ScotchbondTM Universal Etchant, 3M/ESPE, Neuss, Germany) for 30 min and then cleansed
acid with
gel (ScotchbondTM
Universal
Neuss, Germany) fordihydrogen
30 min and
then cleansed
water before being
dried. Etchant,
Following3M/ESPE,
this, 10-methacryloyloxydecyl
phosphate
with(MDP)-containing
water before being
dried.
Following
10-methacryloyloxydecyl
phosphate
primer
(Z-PRIMETM
plus,this,
BISCO,
Schaumburg, IL, USA) wasdihydrogen
coated and lightly
(MDP)-containing
primer
(Z-PRIMETM
plus,aBISCO,
IL, USA) was coated
and lightly
dried with a drier.
We took
and evenly mixed
suitable Schaumburg,
amount of double-polymerized
resin cement
U200,
Germany),
a base agent,
and of
accelerator,
which we then
filled
dried(Rely-XTM
with a drier.
We3M/ESPE,
took andNeuss,
evenly
mixed a suitable
amount
double-polymerized
resin
cement
inside
the
crown.
After
this,
we
applied
a
compressive
force
of
49
N
using
a
static-load
device
to
(Rely-XTM U200, 3M/ESPE, Neuss, Germany), a base agent, and accelerator, which we then filled
cement the crown to the metal abutment. It was then stored in distilled water at 37 °C for 24 h for an
aging treatment.
Materials 2018, 11, 77
11 of 13
inside the crown. After this, we applied a compressive force of 49 N using a static-load device to
cement the crown to the metal abutment. It was then stored in distilled water at 37 ◦ C for 24 h for an
aging treatment.
Materials 2018, 11, 77
11 of 13
4.2.3. Crown Fracture Test
4.2.3. Crown Fracture Test
The prepared crown was fixed vertically on the retainer and then mounted to the universal testing
The prepared crown was fixed vertically on the retainer and then mounted to the universal
machine (4201, Instron, Canton, MA, USA). A steel ball measuring 3 mm in diameter was placed at the
testing machine (4201, Instron, Canton, MA, USA). A steel ball measuring 3 mm in diameter was
crown’s
center and then arranged so as to be placed at the center of the load rod, as shown in Figure 11.
placed at the crown’s center and then arranged so as to be placed at the center of the load rod, as
Then,shown
a compressive
force
was
applied at aforce
crosshead
speedatofa 0.5
mm/min,
and
a load
was recorded
in Figure 11.
Then,
a compressive
was applied
crosshead
speed
of 0.5
mm/min,
and
until athe
moment
when until
a fracture
occurred.
load
was recorded
the moment
when a fracture occurred.
Figure 11. Photograph showing the installation of steel ball and the crown mounted to the universal
Figure 11. Photograph showing the installation of steel ball and the crown mounted to the universal
testing machine for the fracture test of crown.
testing machine for the fracture test of crown.
4.3. Surface Characterization
4.3. Surface Characterization
Using a high-resolution field emission scanning electron microscope (HR FE-SEM, SU8230,
Hitachi,aJapan),
we observedfield
the morphological
microstructure
of the
surface and(HR
bond FE-SEM,
interface after
Using
high-resolution
emission scanning
electron
microscope
SU8230,
the
liner
treatment
on
3Y-TZP
and
the
fractured
surface
after
the
microtensile
test,
after
acid-etching
Hitachi, Japan), we observed the morphological microstructure of the surface and bond interface after
the specimen in a 9% HF aqueous solution for 30 s. Additionally, the surface of the liner-treated 3Ythe liner
treatment on 3Y-TZP and the fractured surface after the microtensile test, after acid-etching
TZP was acid-etched in a 5% HF aqueous solution for 30 min in order to remove a layer of liner, so
the specimen in a 9% HF aqueous solution for 30 s. Additionally, the surface of the liner-treated
as to identify the effect of the liner treatment on the structural change of zirconia grains. We analyzed
3Y-TZP
was acid-etched in a 5% HF aqueous solution for 30 min in order to remove a layer of
the concentration change of elements in the reaction layer at the bond interface using energyliner,dispersive
so as to identify
the effect(EDS,
of the
liner Germany).
treatmentWe
onanalyzed
the structural
change
of zirconia
grains.
X-ray spectroscopy
Bruker,
the crystal
structure
of the bond
We analyzed
the concentration
change
of elements
the reaction
layer(Dmax
at theШ-A
bond
interface
interface after
the liner treatment
on 3Y-TZP
using aninX-ray
diffractometer
type,
Rigaku,using
energy-dispersive
Tokyo, Japan). X-ray spectroscopy (EDS, Bruker, Germany). We analyzed the crystal structure of
the bond interface after the liner treatment on 3Y-TZP using an X-ray diffractometer (Dmax III-A type,
4.4. Tokyo,
Statistical
Analysis
Rigaku,
Japan).
Data were statistically analyzed using the statistical software SPSS ver12.0 (SPSS Inc., Chicago,
4.4. Statistical
IL, USA). Analysis
A one-way analysis of variance (ANOVA) with the Tukey test was performed in order to
compare
thestatistically
bonding strength
between
thethe
zirconia
core and
ceramicSPSS
veneer
and the
singleInc.,
crown’s
Data
were
analyzed
using
statistical
software
ver12.0
(SPSS
Chicago,
fracture strength between the untreated and liner-treated group. We considered a p-value of <0.01 as
IL, USA). A one-way analysis of variance (ANOVA) with the Tukey test was performed in order to
an indication of statistically significant differences.
compare the bonding strength between the zirconia core and ceramic veneer and the single crown’s
fracture
strength between the untreated and liner-treated group. We considered a p-value of <0.01 as
5. Conclusions
an indication of statistically significant differences.
The present study aimed to evaluate the effect of liner treatments of 3Y-TZP ceramic cores on
the bonding strength of lithium-disilicate glass-ceramic veneers and on the fracture strength of
5. Conclusions
bilayered zirconia all-ceramic crowns. To achieve this, we conducted microtensile bond tests and
The
present
to evaluate
effect of crown
liner treatments
of 3Y-TZP
ceramic cores
on the
fracture
testsstudy
on a aimed
bilayered
zirconia the
all-ceramic
and identified
the following
results.
A chemical
occurs via the reaction
layer in the
coupling
between strength
the zirconia
and
bonding
strengthbond
of lithium-disilicate
glass-ceramic
veneers
and portion
on the fracture
of bilayered
lithium-disilicate
liner. Statistically
speaking,
the meanmicrotensile
bonding strength
thefracture
ceramic tests
zirconia
all-ceramic crowns.
To achieve
this, weboth
conducted
bondbetween
tests and
zirconia
and
lithium-disilicate
glass-ceramic
veneer,
and
the
fracture
strength
of
the
liner-treated
on a bilayered zirconia all-ceramic crown and identified the following results. A chemical bond
group were significantly higher than those of the untreated groups. We confirmed that for the
untreated group, the resistance to fractures dropped due to the stress concentration at the interface,
Materials 2018, 11, 77
12 of 13
occurs via the reaction layer in the coupling portion between the zirconia and lithium-disilicate
liner. Statistically speaking, both the mean bonding strength between the ceramic zirconia and
lithium-disilicate glass-ceramic veneer, and the fracture strength of the liner-treated group were
significantly higher than those of the untreated groups. We confirmed that for the untreated group,
the resistance to fractures dropped due to the stress concentration at the interface, which resulted from
the generation of a microgap between the core and veneer. For the liner-treated group, on the other
hand, the resistance to fractures rose due to the stress relaxation at the interface, which resulted from
the elimination of microgap between the core and veneer through the binding of the veneer and liner.
In conclusion, given that the chemical bonding occurs at the interface, the application of a
lithium-disilicate liner to zirconia and lithium-disilicate glass-ceramic veneering can greatly contribute
to improvements in the fracture strength of crowns.
Acknowledgments: This research was supported by the Technological Innovation R&D Program (S2410617)
funded by the Small and Medium Business Administration (SMBA, Korea) and Fund of Biomedical Research
Institute, Chonbuk National University Hospital.
Author Contributions: Tae-Sung Bae, Myung-Jin Lim and Hyeong-Rok Noh conceived and designed the
experiments; Hyeong-Rok Noh performed the experiments; Yong-Seok Jang, Min-Ho Lee and Myung-Jin Lim
analyzed the data; Yong-Seok Jang, Hyeong-Rok Noh and Tae-Sung Bae cooperatively wrote this article.
Conflicts of Interest: The authors declare no conflict of interest.
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