Pressed ceramics onto zirconia. Part 1: Comparison of crystalline phases present, adhesion to a zirconia system and flexural strength

d e n t a l m a t e r i a l s 2 7 ( 2 0 1 1 ) 1204–1212

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Pressed ceramics onto zirconia. Part 1: Comparison of crystalline phases present, adhesion to a zirconia system and flexural strength Jung Eun Choi, J. Neil Waddell, Brendan Torr, Michael Vincent Swain ∗ Sir John Walsh Research Institute, Faculty of Dentistry, University of Otago, Dunedin, New Zealand

a r t i c l e

i n f o

a b s t r a c t

Article history:

Objectives. To compare the crystalline phases present, quantify the adhesion to zirconia and

Received 13 October 2010

measure the mechanical properties of four commercially available pressed ceramics suitable

Received in revised form

for zirconia substructures.

14 June 2011 Accepted 17 August 2011

Materials and methods This study compares the X-ray diffraction response and the mechanical properties of four different pressed ceramics (Noritake CZR Press, Vita PM9, Wieland PressXzr and IPS e.max ZirPress) to Vita In-Ceram YZ zirconia substrate. The adhesion was determined using the interfacial strain energy release rate fracture mechanics

Keywords:

approach; in addition biaxial flexural strength values of each material was determined.

Zirconia

Results. X-ray diffraction analysis revealed that Noritake CZR Press and Vita PM9 contain

All-ceramic restorations

leucite whereas IPS e.max ZirPress and Wieland PressXzr are non-leucite amorphous mate-

Pressed ceramics

rials.

X-ray diffraction

The strain energy release rate results revealed that the pressed ceramics with leucite have

Adhesion

better adhesion than non-leucite ceramics to zirconia. Differences were observed between

Flexural strength

biaxial strength results for the pressed ceramics from bilayer compared with monolayer specimens. Conclusions. Pressed ceramics compatible with zirconia tested in this study were of two types; leucite containing and non-leucite containing essentially glass ceramics. Leucite containing pressable ceramics appears to have better adhesion to zirconia. Crown Copyright © 2011 Published by Elsevier Ltd on behalf of Academy of Dental Materials. All rights reserved.

1.

Introduction

Esthetic demands from patients and clinicians are leading to increase usage of all-ceramic restorations instead of porcelain-fused-to-metal (PFM) and full metal crown restorations. This has resulted in a large number of all-ceramic systems becoming available on the dental market. Due to this diversity, appropriate material selection in clinical situations for treating unesthetic teeth has become difficult.

∗

The impressive properties of advanced oxide ceramics has enabled their utility as core structures and usage in more demanding applications where higher stresses are anticipated rather than the more traditional porcelain and metal-ceramics. Yttria tetragonal zirconia polycrystalline (YTZP) materials, in particular, are becoming more popular core ceramics due to their high strength of 900–950 MPa and high elastic modulus 200 GPa. The most common technique to manufacture all-ceramic restorations is to brush-applied veneer the porcelain onto

Corresponding author. Tel.: +64 3 479 4196; fax: +64 3 479 5079. E-mail addresses: [email protected], [email protected] (M.V. Swain).

0109-5641/$ – see front matter Crown Copyright © 2011 Published by Elsevier Ltd on behalf of Academy of Dental Materials. All rights reserved.

doi:10.1016/j.dental.2011.08.006

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Table 1 – Summary of the materials used in the study with their coefficient of thermal expansion and glass transition temperatures. Materials used

CTE (ppm/◦ K)

Manufacturer

Pressed ceramics Vita PM9 IPS e.max ZirPress Wieland Xzr Noritake CZR Zirconia Vita InCeram-YZ

Tg (◦ C)

Vita Zahnfabrik Ivoclar Vivadent Wieland Dental + Technik Noritake Kizai Co.

9.2 9.75 9.3 10.1

640 530 620 615

Vita Zahnfabrik

10.5

N/A

sintered ceramic cores. Recently a heat pressing technique has become available to produce a veneering layer for allceramic restorations [1]. Anecdotal evidence and marketing information from dental companies have reported that dental technicians prefer the pressing technique because of its speed, accuracy and stability [1]. In addition framework supported wax patterns can be tried in the mouth enabling adjustments before pressing and sintering without influencing their mechanical properties [2]. In comparison the sintering process is more technique sensitive than pressing because of the brush-applied build-up and firing techniques [3]. However, heat pressing materials have relatively low strength and fracture toughness which limits their use to conservative designs in low to moderate stress environments [1]. Despite the growing use of zirconia some clinical studies are reporting veneering failure; namely chipping of the veneering porcelain as a major issue. According to Sailer et al.’s study, the chipping of veneer is the most frequent reason for failure with a failure rate of 15.2% after an in-service time of 35.1 ± 13.8 months [4]. Possible reasons for chipping are; insufficient bond strength, excessive tensile stress due to a CTE mismatch, excessive load due to premature contacts, insufficient substrate support, tensile stress established during cooling after firing, especially when a considerable thermal gradient develops through the layered system upon rapid cooling [5,6]. Chipping and fracture issues have also been reported with pressed veneering ceramics. Christensen et al. compared the failure rate of PFM restorations with Y-TZP based ceramics

both pressed and hand-layered. According to their report, pressed ceramics have a lower failure rate than hand layered restorations after 2 years [7]. This supports Taskonak et al.’s observations comparing pressed to layered veneering over zirconia and metal, where pressed ceramics performed significantly better [8]. Christensen et al. however found that the failure rate of IPS e.max ZirPress, which was as high as other brush-applied all-ceramic systems, was double that of Noritake’s CZR press material. Moreover, both the brush-applied veneered zirconia system and the pressed to zirconia system had much higher failure rates than PFM restorations. In addition, the most common type of fracture for pressed ceramics to zirconia was chipping of the porcelain, similar to that found in the brushapplied veneered porcelains. Currently the origin of differences in failure rate between products and between pressed and brush-applied veneered systems is unclear; whether it is formulation differences, difference in intrinsic properties, or the influence of cooling rate as proposed for the brush-applied veneered system. Therefore an investigation of such properties of pressed materials may contribute toward a solution of the high failure rate and chipping associated with Y-TZP based all-ceramic restorations. The present study has two primary objectives, which were to compare the crystalline phases present of four different pressed ceramics for zirconia to evaluate the mechanical properties of four different pressed ceramics for zirconia; porcelain–zirconia adhesion and flexural strength.

Table 2 – Pressing and glazing schedule for the pressed ceramics used in the study. (a) Pressing schedule Brand/name of product

Vita/PM9 Noritake/CZR Press IPS e.max ZirPress Wieland/PressXzr

Heat up temp (◦ C) 850 850 900 900

Start temp (◦ C) 700 700 700 700

Heat rate (◦ C/min)

Vacuum hold time (min)

50 60 60 60

20 26 15 20

Pressing temp (◦ C) 1000 1065 910 1060

Press time (min) 6 6 6 8

(b) Glazing schedule Brand/name of product Vita/PM9 Noritake/CZR IPS e.max ZirPress Wieland/PressXzr

Drying time (min) 5 5 5 5

Start temp (◦ C) 500 600 450 575

Heat rate (◦ C/min) 80 65 60 75

Firing temp (◦ C) 900 900 770 880

Hold time (min) 1 1 1 1

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In a companion paper the indentation fracture toughness and the influence of glaze firing conditions on the residual stress development in pressed veneering porcelains are investigated (Choi et al.).

2.

the ceramic plate with water cooled diamond impregnated saw (Hamco Machines Inc., Rochester, NY). A small pre-crack was created at the porcelain–zirconia interface at the base of the notch then stored in kerosene to minimize the influence of water vapor on crack growth during testing [12]. The specimens were tested in a Universal Testing Machine (Instron 3369) in a 4-point bending jig at a crosshead speed of 0.05 mm/min. Load and displacement curves were recorded and all contained a plateau which represents the critical load to propagate the crack for interface fracture. The mean value was used to calculate the strain energy release rate (G) using the following formula:

Materials and methods

The pressed veneering materials used in this study are listed in Table 1.

2.1.

Sample preparation

Vita In-Ceram YZ blocks were cut with Automan 36 (Struers, Denmark) to produce zirconia core for samples using a diamond blade. The zirconia blocks were cut 20% bigger than the desired dimension considering its shrinkage. The pressed ceramics were prepared to the dimensions required for each test following the manufacturers’ instructions (see Table 2(a)). The samples were wet ground and polished to the desired dimension for each test on one surface only, using 120–4000

=

3 2

1 (hz /h)

3

−



3

3

[(hz /h) + (hz /h) + 3(hp hz /h2 )/[(hp /h) + (hz /h)]]

X-ray diffraction

The materials used in this analysis were divided into 3 groups; before pressing, after pressing and after glazing. For the first group, as received ingots were cut into 2 mm thick disks and the ground surfaces of the disks X-rayed. For group 2; after pressing ceramics were ground into disks 14 mm × 2 mm. 1 disk from each group was randomly selected to be glazed. A total of 12 disks; 3 per material group were analyzed by Xray diffraction. Samples were scanned from 10◦ to 80◦ 2 at 0.05◦ steps with Cu radiation (Phillips, PANalytical X’Pert Pro MPD). The phase identification was carried out using the JPD database and HighProData software.

2.3.

(1)

where P is the load to induce stable crack growth, l is the moment arm or distance between inner (12 mm) and outer load line (24 mm)(rollers) on the same side, z (0.3) and Ez (200 GPa) are Poisson’s ratio and elastic modulus of zirconia substructure, respectively, and b and h are the specimen width and total thickness, respectively. The non-dimensional parameter  is calculated with all specimen dimensional terms by 

grades of abrasive paper (Struers PSA backed Silicon carbide paper) on a metallographic lapping machine (Knuth Roter, Struers, Denmark). All samples were glazed following the schedule indicated in Table 2(b) using a Dekema furnace (Dekema Astromat M).

2.2.

[P2 l2 (1 − z2 )] Ez b2 h3

G=

Porcelain–zirconia adhesion test

The interfacial fracture mechanics approach developed by Charalambides et al. was used to evaluate the adhesion of porcelain to Y-TZP as previously used to evaluate the adhesion of porcelain to metal in PFM systems [9,10]. This approach measures the strain energy release rate associated with stable crack extension at the interface between bi-material plates. The various porcelains (n = 10) were pressed on to zirconia plates (8 mm × 1.5 mm × 30 mm) and ground on the porcelain side to a plate thickness of 3 mm. Following the approach developed by Suansuwan and Swain [11] specimens were notched to the porcelain–zirconia interface in the middle of

And =

Ez (1 − z2 ) Ep (1 − p2 )

where p (0.25) and Ep (70 GPa) are Poisson’s ratio and elastic modulus of porcelain and hp and hz are thickness of porcelain and zirconia, respectively. The values for all groups were analyzed and compared by conducting T-tests. SEM (Stereoscan 360, Cambridge Instruments) images were taken to characterize fracture patterns. Two representative specimens of the fracture samples were selected from each group for SEM analysis.

2.4.

Bi-axial flexural strength test

The bi-axial fracture strength of ten bi-layer disk specimens per group was determined using the piston on three-ball test. After the porcelain was pressed on to the zirconia samples (17.4 mm × 17.4 mm × 1.1 mm), the plates were ground to realize a plate thickness of 2.2 mm. Samples were stored in kerosene prior to testing. The porcelain of the disk specimens was placed in tension during biaxial flexural strength testing. A thin plastic sheet (0.05 mm) was positioned between the loading cylinder (1.58 mm diameter) to distribute the load evenly. The three balls supporting the plates were located at 4 mm radius. Specimens were tested in a Universal Testing Machine (Instron 3369) at a crosshead speed of 0.05 mm/min. The following equations were used to determine the flexure strength [13].

Zn =

Ep h2p /2(1 − p2 ) + Ez h2z /2(1 − z2 ) + Ez hp hz /(1 − z2 ) Ep hp /(1 − p2 ) + Ez hz /(1 − z2 )

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Fig. 1 – X-ray diffraction patterns of the four pressed materials studied: (a) Noritake CZR Press before pressing, (b) Vita PM9 before pressing, (c) Vita PM9 after pressing, (d) Wieland PressXzr after pressing and (e) IPS e.max ZirPress after pressing.

D=

Ep h3p 3(1 − p2 ) −

+

Ez h3z 3(1 − z2 )

+

The intrinsic properties of the monolithic pressed materials were bi-axially tested using the piston on three-ball test. The distance from the center to three balls was 6 mm and specimens were 2 mm thick and 20 mm in diameter. The following formula was used:

Ez hp hz (hp + hz ) (1 − z2 )

[Ep h2p /2(1 − p2 ) + Ez h2z /2(1 − z2 ) + Ez hp hz /(1 − z2 )]

2

Ep hp /(1 − p2 ) + Ez hz /(1 − z2 )

=

p hp + z hz = hp + hz



=

−PEp (Z − Zn) 2 ln 8(1 − p )D

a b

+

(1 − )(a2 − b2 ) (1 − )R2

 (2)

For 0 ≤ Z ≤ hp and R ≤ b. where Ep is the elastic modulus of porcelain, Ez the elastic modulus of zirconia, hp is the thickness of porcelain, hz the thickness of zirconia, p is the Poisson’s ratio of porcelain, z the Poisson’s ratio of zirconia, a is the radius of the supporting circle and b is the radius of the tip of the piston, R is the radius of the specimen and P is the load to fracture. Statistical T-test was used to compare the collected data.

−0.238P(X − Y) h2

(3)

where X = (1 + )ln(B/C)2 + [(1 − )/2](B/C)2 , Y = (1 + )[1 + ln(A/ C)2 ] + (1 − )(A/C)2 ,  flexural strength (MPa), P is the load at fracture, h the specimen thickness,  the Poisson’s ratio, A is the radius of the support circle, B is the radius of the tip of the piston and C is the radius of the specimen.

3.

Results

3.1.

X-ray diffraction

The X-ray diffraction results are shown in Fig. 1a–e. All the pressed materials had a dominant amorphous response with a broad peak over the 2 interval from 15◦ to 35◦ . For the

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Table 3 – Summary of adhesion (G) and flexural strength results (SD – Standard Deviation). Strain Energy release rate, G (J/m2 ) (SD)

Noritake CZR Press Vita PM9 Wieland PressXzr IPS e.max ZirPress

26.7 (2.1) 21.3 (2.7) 17.2 (2.6) 17.1 (3.2)

Noritake CZR Press and Vita PM9 pressed materials there were clearly identifiable minor crystalline phases present whereas the Wieland PressXzr and IPS e.max ZirPress materials were primarily amorphous. The major crystalline phase present in Noritake CZR is leucite (Fig. 1a). No additional peaks were observed after any of the processing steps (pressing and glazing). The leucite found before and after pressing was tetragonal in structure. For Vita PM9 the major peak was leucite (Fig. 1b and c). The leucite identified in the material before pressing was also tetragonal. After pressing, there was a significant decrease in the peak intensity and a further slight decrease after glazing. Moreover, the trace found after pressing and glazing had slightly different peak positions consistent with cubic rather than tetragonal structure as found before pressing. For Wieland PressXzr (Fig. 1d) and IPS e.max ZirPress (Fig. 1e), the diffraction patterns demonstrated a broad amorphous scatter with minimal evidence of specific crystalline phases present.

3.2.

Bilayer

Monolayer

122.6 (6.5) 132.8 (11.4) 103.7 (12.5) 93.2 (4.4)

54.0 (16.0) 89.2 (8.2) 66.9 (11.0) 89.6 (16.2)

that the Vita PM9 had the highest flexural strength, Noritake being the second highest and Wieland and IPS e.max followed. The leucite containing ceramics have higher strength values than non-leucite containing ceramics (Table 3). The T-tests of results revealed that there is no statistically significant difference between the flexural strength when the materials with the same crystalline content were compared; Noritake vs. Vita material (the leucite containing materials) and Wieland vs. IPS e.max material (glass ceramics). However, there is a statistically significant difference when leucite ceramics and glass ceramics were compared. The examination of the disk specimens after fracture showed most had broken into 3 and more segments whereas some specimens broke into 2 pieces. This was evident in all material samples tested. For Noritake, the specimens broke into 2 or 3 pieces but for all samples the porcelain stayed on zirconia, where as all Vita samples broke into 3 or more pieces but for most samples, the veneered porcelain fractured off. For Wieland and Emax, most samples broke into 3 or more pieces and the veneered porcelain stayed on for some samples and fractured off for other tests.

Pressed veneer-zirconia adhesion

The interfacial strain energy release rate (G) for each pressed ceramic was calculated using Eq. (1), and is presented in Table 3. It shows that the leucite containing pressed ceramic systems have higher values than non-leucite materials. TTests revealed that there is a significant difference between the leucite and non-leucite ceramics. The failure mode observed by SEM was cohesive failure within the porcelain for all systems examined (Figs. 2 and 3). In no instance did the porcelain of the Noritake CZR group separate from the Y-TZP completely, whereas for the other three products, in most instances, part of the porcelain separated from the specimen. The Noritake CZR material was found to be very homogeneous, with no obvious porosity. SEM images of the fractured surface of Vita PM9 showed classic brittle fracture pattern. Vita PM9 had a relatively homogeneous fracture compared to the Noritake material. The crack in this sample propagated without branching (Fig. 2d–f). Like Vita PM9, IPS e.max ZirPress (Fig. 3a–c) samples crack propagated with no branching. Under SEM, it was found that the Wieland PressXzr had the highest porosity compared to the other pressed ceramics.

3.3.

Biaxial flexural strength, MPa (SD)

Flexural strength test

The bi-axial flexure strength results from Eq. (3) for monolayer samples are listed in Table 3. IPS e.max ZirPress and Vita PM9 were found to have the highest strength values with 89.6 and 89.2 MPa, respectively, while Noritake had the lowest with 54.0 MPa. For the bi-layered samples from Eq. (2), it was found

4.

Discussion

4.1.

X-ray diffraction

The X-ray diffraction results have shown that some pressed ceramics for zirconia contain leucite; Noritake CZR Press and Vita PM9, while some primarily consist of amorphous glass; Wieland PressXzr and IPS e.max ZirPress. For IPS e.max ZirPress, the X-ray diffraction pattern was in good agreement with Tsalouchou et al.’s study who also found that the X-ray diffraction pattern of IPS e.max ZirPress was predominantly amorphous glass with minimal discernable presence of fluorapatite [3]. Comparison of the Noritake (Fig. 1a) and Vita material (Fig. 1b) before pressing indicated, a higher peak count obtained for leucite in Noritake CZR Press, implying a slightly higher volume fraction compared to that of Vita PM9. In addition, the X-ray analyses revealed the leucite in both materials before pressing were tetragonal. For Noritake the ratio of the leucite to glass decreased slightly upon pressing and glazing. At room temperature, leucite crystals are tetragonal, but when heated the crystalline structure changes to cubic at 625 ◦ C and undergoes the reverse transformation upon cooling [14]. This decrease in peak count for leucite may have been caused by slight dissolution of the leucite in the glass matrix at elevated temperatures that was not reprecipitated fully upon cooling after pressing [2]. The peaks present in the X-ray traces of Vita material after pressing were less readily identifiable due to the lower

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Fig. 2 – SEM images of Noritake and Vita material after subjected to adhesion test. (a) The origin of the crack induced by pre-cracking near the notch. (d) The overview of Vita sample showing the notch and the cracks induced from pre-cracking. (b and e) Crack propagation shown on the Noritake (b) and Vita (e) sample, respectively. (c and f) Close up of the fractured surface of Noritake and Vita material, respectively, showing typical porcelain–porcelain brittle fracture.

volume fraction of the crystalline structure. When these peaks were compared it was found that the peaks after pressing matched better with the cubic leucite structure. This may be caused by the transformation of leucite crystals from cubic to tetragonal phase at elevated temperature that was not able to retransform upon cooling. The above mentioned leucite phase transformation not only affects the structural arrangements but also the coefficient of thermal expansion (CTE) and the mechanical properties of the porcelain as initially found by Mackert and Evans [15]. Cesar et al. and Kontonasaki et al. suggest that changes in structure cause thermal contraction mismatch between the leucite and the glass matrix, which contributes to the toughening mechanism of leucite containing ceramics [14,16]. Kontonasaki et al. also stated that the leucite content determines the CTE of the final product. Therefore the X-ray diffraction patterns obtained can be linked to the CTE of

Noritake CZR and Vita PM9 with the latter being potentially lower after pressing [16]. The CTE of Noritake CZR is 10.1 ppm/◦ K whereas that of Vita PM9 is 9.2 ppm/◦ K, creating a difference of 0.9 ppm/◦ K, which is significant, especially when they are used for the same core structure. The larger volume fraction of leucite in Noritake material may have resulted in it having a higher CTE than that of Vita material. In addition, Kontonasaki et al. asserted that the leucite volume fraction may be altered due to repeated firings and different cooling rates [16]. According to Isgro et al., additional uncontrolled crystallization may occur during firing procedures employed in their manufacturing process resulting in a change in the thermal expansion behavior of the materials [17]. In addition a recent study by Zhang et al. showed that solubility and precipitation of the leucite phase may occur at temperatures above 800 ◦ C thus supporting the basis for the changes in leucite content observed in the

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Fig. 3 – SEM images of IPS e.max ZirPress (a–c) and Wieland (d–f) material after subjected to adhesion test. (a) The overview of IPS e.max ZirPress sample showing the notch and the cracks induced from pre-cracking. (d) The origin of the crack induced by pre-cracking near the notch in Wieland sample. (b and e) Crack propagation shown on the IPS e.max and Wieland sample. (c and f) Close up of the fractured surfaces of IPS e.max and Wieland material, respectively.

present work [18]. These authors also showed that metastable cubic leucite crystals exist upon cooling to room temperature provided their size was less than 50 nm.

4.2.

Pressed porcelain–zirconia adhesion

The interfacial strain energy release rate (G) in Table 3, showed that the leucite containing pressed ceramic systems have higher adhesion than that of non-leucite ceramic systems. TTests revealed that there is a significant difference between the G value for leucite and non-leucite veneering ceramics. Noritake’s pressed ceramic for zirconia showed the highest value among the 4 tested materials and corresponded to the material with extensive crack branching as shown in Fig. 2b. There are several variables associated with adhesion of two different materials including intrinsic bonding, CTE mismatch as well as the presence of other residual stresses [13,19]. For bonded ceramic systems, the CTE mismatch and the residual

stress formation is thought to influence adhesion. Previous studies indicated that the material with high CTE (i.e. lower CTE mismatch to the core) are found to be stronger [21]. Out of the four tested pressable ceramics, Noritake and Vita were found to contain leucite as the main crystalline structure by X-ray diffraction analysis. The CTE of Noritake CZR is higher than that of Vita PM9 (10.1 and 9.2 ppm/◦ K, respectively) which better matches the CTE of the zirconia (10.5 ppm/◦ K). This may have resulted in higher strain energy release rate of Noritake than Vita PM9 when the same core structures were used for both. However for the two glass-ceramics; Wieland PressXzr and IPS e.max ZirPress, there was no significant difference found between the adhesion or G values of the two materials despite IPS e.max ZirPress having a higher CTE mismatch to the core than the Wieland material. A simpler explanation may be that as the fracture path in all instances occurs within the veneering porcelain and the intrinsic toughness of leucite containing pressed veneering materials is higher than

d e n t a l m a t e r i a l s 2 7 ( 2 0 1 1 ) 1204–1212

the non-leucite porcelains with the strain energy release rate reflecting the intrinsic toughness of the pressed porcelains. This interpretation is in agreement with Cesar et al.’s results, where they claim that CTE mismatch does not influence adhesion strength [21]. The correlation between the fracture toughness and the leucite content of porcelain is another factor influencing adhesion [14]. The higher the amount of leucite, the higher is the resistance to crack propagation [21]. The analysis performed showed that crack deflection upon interacting with a second phase is the main toughening mechanism acting in leucite based porcelains. This was most evident for Noritake samples (Fig. 2) where SEM images of fracture surface were very rough demonstrating that crack deflection occurred. Despite considerable literature [6,23,24] on the factors affecting the adhesion of porcelain and zirconia, the bonding mechanisms are still unclear. Based on the wettability of veneering ceramics onto Y-TZP, micromechanical interactions were assumed [21]. According to Ban et al., there was no evidence for chemical bonding between zirconia and porcelains because SEM observations could not confirm the presence of a reaction layer between the zirconia and the veneering porcelain after sintering [25]. However, as with metal-ceramic systems, chemical bonding across the interface of zirconia and glass is achieved by having thermodynamic equilibrium at the interfacial zone and with some degree of solubility in the veneering porcelain glass of zirconia ions. In addition the low viscosity of the glass phase and its excellent wetting ability of zirconia would enable some degree of penetration of the glass along Y-TZP grain boundaries. The failure mode of mainly cohesive fracture within all the bonded pressed porcelains of this study was in contrast with most previous literature. In Aboushelib et al.’s study, the failure mode of Cercon core-veneer and Vita core-veneer specimens was stated as predominantly interfacial [21]. Whereas, in Cesar et al.’s study, the failure modes showed more mixed results [22]. When Cercon base zirconia and Vita YZ and veneering ceramics were tested, the failure modes observed were combined adhesive at the interface and cohesive in the veneering ceramics. The difference between the results may have arisen as those studies used shear bond or micro-tensile bond strength tests. The failure mode observed in this study is more in agreement with clinical results; where the fracture of the zirconia veneering systems with chip-off fractures within the veneering porcelain were observed rather than the failures at the interface [16].

4.3.

Flexural strength test

To the knowledge of the authors, no systematic investigation of the flexural strength of pressed ceramics for zirconia is available; Fisher et al. studied the flexural strength of hand layering ceramics to zirconia. However, mono-layered specimens were used in their study rather than bi-layered when the samples were subjected to biaxial flexural strength testing [26]. The results found for mono-layered samples (Table 3) were in a similar range with Fisher et al.’s study. In their study, the bi-axial flexural strength of hand layered ceramic to zirconia was found to be in the range of 50–80 MPa [24]. The fact that

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the strength of veneering ceramics for zirconia is in the same range as that of veneering ceramics for PFM reveal that the flexural strength is not the limiting factor for the clinical longterm success of zirconia restorations. Fisher et al. also stated that despite this similarity with veneering ceramics for PFMs, excessive chipping is observed in clinical studies with zirconia restorations, which emphasizes the effect of stress built up during cooling, especially rapid cooling, and the rigidity of the zirconia substructure [5,26]. In this study, it was found that the leucite containing ceramics have higher bilayer strength values than non-leucite containing ceramics. This is in agreement with Cattell et al.’s research where they found that the leucite reinforced ceramics produced higher strength values than the feldspathic porcelain. According to Albakry et al., leucite containing ceramics have higher strength since the leucite grains within the glass reduce flaws [20,27]. The leucite content is also closely related to the bilayer strength of the ceramic system but not for the monolayer samples. The work of Kon et al. however showed that the addition of high quantities of leucite led to a significant reduction in the strength values of the porcelain tested compared to specimens with lower leucite contents. They also found that a porcelain with 40% leucite content had significantly lower strength compared to porcelains with contents of 0–25% [19]. This is in accord with the monolayer Noritake material but not so for the Vita material although the latter has a smaller volume fraction of leucite crystals. However, some materials with lower volume fraction of leucite were found to have higher flexural strengths such as Vita material.

5.

Conclusions

Within the limitations of this study, the following conclusions may be drawn:

X-ray diffraction analysis revealed that Noritake CZR Press and Vita PM9 are leucite containing ceramics whereas IPS e.max ZirPress and Wieland PressXzr are non-leucite amorphous glass ceramics. The adhesion of zirconia to pressed ceramics with leucite has higher strain energy release rate values than non-leucite ceramics. SEM results showed that fractures were primarily cohesive failure within the porcelain when it was pressed on to zirconia. This suggests delamination due to adhesive failure between zirconia and the veneering porcelain is not the major issue responsible for chipping of ceramic veneers. The leucite ceramic bilayer systems have higher bilayer flexural strength than the non-leucite ceramics.

Acknowledgements The authors wish to thank Dr. Basil Al-Amleh for his critical reading of the paper. We thank Ms. Liz Girvin of The Otago Centre for Electron Microscopy, and also Damian Walls of the Department of Geology, University of Otago.

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d e n t a l m a t e r i a l s 2 7 ( 2 0 1 1 ) 1204–1212

references

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