d e n t a l m a t e r i a l s 2 4 ( 2 0 0 8 ) 1077–1082
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Fractographic analyses of zirconia-based fixed partial dentures Burak Taskonak a,∗ , Jiahau Yan a , John J. Mecholsky Jr. b , ¨ c , Ays¸e Koc¸ak c Atilla Sertgoz a
Department of Restorative Dentistry, Indiana University School of Dentistry, Indianapolis, IN, USA Department of Materials Science and Engineering, College of Engineering, University of Florida, Gainesville, FL, USA c Department of Prosthodontics, Marmara University College of Dentistry, Istanbul, Turkey b
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objectives. Advances in ceramic processing techniques enable clinicians and ceramists to
Received 22 June 2007
fabricate all-ceramic fixed partial dentures (FPDs) for posterior regions using high-strength
Received in revised form
yttria-stabilized tetragonal zirconia polycrystals (Y-TZP). However, failures occur in ceramic
10 December 2007
FPDs due to their design. The objectives of this study were to determine the site of crack
Accepted 24 December 2007
initiation and the causes of fracture in clinically failed zirconia-based ceramic FPDs. Methods. Five clinically failed four-unit Y-TZP-based FPDs (Cercon® ceramics, DeguDent GmbH, Hanau, Germany) were retrieved and analyzed. The fragments containing the frac-
Keywords:
ture origins in the veneers (Cercon® Ceram S Veneering Ceramic, DeguDent GmbH, Hanau,
Dental ceramic
Germany) of two samples were missing but the rest of veneer structures were present.
Fracture surface analysis
The other three samples had their veneers intact. Fracture surfaces were examined using
Fractography
fractographic techniques, utilizing both optical and scanning electron microscopes (SEM).
Zirconia
Quantitative fractography and fracture mechanics principles were used to estimate the
Fracture mechanics
stresses at failure. Results. Primary fractures initiated from the gingival surfaces of connectors at veneer surfaces in four out of the five samples. However, critical flaw sizes could be measured in three of the five cases since fracture origins were lost in the remaining two due to local fragmentation at the crack initiation site. Delaminations between glass veneer and zirconia core were observed in Y-TZP-based FPDs and a secondary fracture initiated from the zirconia core. Secondary fracture controlled the ultimate failure. Failure stresses of the fixed partial dentures that failed due to zirconia fracture ranged from 379 to 501 MPa. Fractures that had origins on the glass veneer surface had failure stresses between 31 and 38 MPa. Significance. Primary fractures in clinically failed Y-TZP-based FPDs initiated from the veneer surfaces. Interfacial delamination in glass veneer/zirconia core bilayer dental ceramic structures controlled the secondary fracture initiation sites and failure stresses in Y-TZP-based fixed partial dentures. © 2008 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
∗ Corresponding author at: Department of Restorative Dentistry, Division of Dental Biomaterials, Indiana University School of Dentistry, 1121 West Michigan Street, Indianapolis, IN 46202, USA. Tel.: +1 317 274 3725; fax: +1 317 278 7462. E-mail address:
[email protected] (B. Taskonak). 0109-5641/$ – see front matter © 2008 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.dental.2007.12.006
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1.
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Introduction
Fixed partial dentures (FPDs) can be fabricated using metals, ceramics or ceramics (porcelain) fused to metals. However, in recent years ceramics are becoming the materials of choice for cases where aesthetics is one of the primary expectations [1]. All-ceramic FPDs are fabricated using a core ceramic, e.g., glass infiltrated alumina, lithia-disilicate glass ceramic, densely sintered aluminum oxide, or yttria-stabilized tetragonal zirconia, veneered with silicate glass to enhance the aesthetic outcome. The most recently developed core ceramic is the yttria-stabilized tetragonal zirconia polycrystals (Y-TZP). The clinical use of the high toughness and strength Y-TZP in multi-unit FPDs has increased since it was introduced to the dental community [2,3]. Compared to previously used core ceramics, Y-TZP presents superior fracture toughness (9–10 MPa m1/2 ), primarily due to the stress-induced phase transformation mechanism [4,5], and hence a longer survival time is expected. The use of Y-TZP as a structural framework enables clinicians to fabricate all-ceramic posterior FPDs [6–9]; however, long-term clinical survival data of these prostheses is not currently available [1]. Ceramic materials are susceptible to fracture under tensile or flexural loading and high stresses can develop at the gingival side of the connector during occlusal loading [10,11]. A previous study showed that 50% of lithia-disilicate core ceramic FPDs failed due to fractures of the connectors [12]. It was recommended in another study that such FPD failures can be prevented by fabricating connectors with sufficient height and width and by reducing the stress concentration areas [13]. Another factor that decreases the clinical survival of all-ceramic FPDs is subcritical crack growth (SCG) [14]. SCG is usually caused by environmentally enhanced crack propagation at stress intensity factor (KI ) levels less than the critical stress intensity factor (KC , also called fracture toughness) [15–17]. Long-term or repetitive low-level loading may also cause pre-existing subcritical flaws to slowly grow until failure occurs at a loading level that is insufficient to cause failure of the virgin dental ceramic prostheses. Because of the aforementioned detrimental factors, failure analysis of Y-TZP supported ceramic FPDs is both essential and important. Fracture surface analysis or quantitative fractography can be used to identify the cause of failure, the stress at failure and
Fig. 2 – Fractured connector in a posterior Y-TZP-based fixed partial denture. Arrow indicates the fracture line.
the existence or not of SCG by characterizing features on the fracture surface, in conjunction with fracture mechanics principles. Fractographic analysis can be a critical tool in analyzing failure mechanisms, identifying fracture initiation sites, and determining the probable causes of the failures in retrieved clinical specimens. Thus, fractographic analysis should be one of the key elements in the design and development of dental structural materials [18]. This paper addresses the application of fractography to Y-TZP-based ceramic FPDs, and the “trouble-shooting” of in-service failures. The overall objectives of this study were to identify, by fractographic techniques, the principal crack initiation sites, failure stresses and the causes of clinically failed Y-TZP-based fixed partial dentures.
2.
Quantitative fractography was used to determine the failure stresses of five veneered four-unit Y-TZP-based fixed partial dentures (Cercon® ceramics, DeguDent GmbH, Hanau, Germany) (Fig. 1). X-ray diffraction analyses showed that the veneering ceramic consisted of amorphous glass. The clinically failed Y-TZP FPDs were retrieved from a clinical study (Fig. 2).
2.1.
Fig. 1 – Schematic of a four-unit Y-TZP fixed partial denture.
Materials and methods
Sample preparation
The first and foremost element of a good fractographic analysis is a clean surface. The fracture surfaces of the specimens were cleaned using ethyl alcohol in a sonicated bath. After removal from the sonicated bath, specimens were rinsed with distilled water. Acetone and methanol were not used because they often leave a film on the surface [19]. After cleaning, fracture surfaces of the specimens were coated with a carbon or gold palladium film (approximately 10–15 nm in thickness) to accommodate better observation. Carbon coating should be used if energy dispersive analysis is desired to detect elemental information.
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Table 1 – Critical crack size, failure stresses and connector dimensions in buccolingual (BL) and occlusogingival (OG) directions of clinically failed Y-TZP-based FPDs #1
#2
#3
#4
#5
Average ± S.D.
Missing N/A
279 ± 55 34 ± 3
121 476
133 ± 34 461 ± 49
Veneer Crack size (m) f (MPa)
222 38
284 34
331 31
Missing N/A
Connector dimensions with veneer layer (BL × OG) (mm)
4.3 × 4.5
6.3 × 4.0
6.1 × 4.0
6.8 × 4.2
Y-TZP Crack size (m) f (MPa)
193 379
110 501
126 459
113 492
Connector dimensions without veneer layer (BL × OG) (mm)
2.9 × 3.0
3.5 × 3.2
3.7 × 2.3
3.7 × 2.2
2.2.
Observation
A hand lens (6×) was first used to observe the fracture surfaces before any quantitative characterization using optical or scanning electron microscopes (SEM). In many cases, characteristic brittle fracture patterns (e.g., macroscopic twist hackle) could be readily seen on the fracture surfaces of the veneers. After examination by the aided eye, a low-power optical microscope was used for overall examination and to confirm the fracture origins. A high-power optical microscope was then used to quantitatively measure the fracture initiating crack dimensions. An SEM was also used to confirm the fracture origins and to measure the crack dimensions. In addition, connector dimensions of the failed FPDs were measured in bucccolingual (BL) and occlusogingival (OG) directions at the fracture surface with and without their veneer layers (Table 1).
2.3.
Analysis
Our first step in failure analysis of FPDs was to locate the origin by looking for a relatively flat region that is often a good indication of the location of fraction origin. We used characteristic markings to trace back to the fracture origin. These are markings on the fracture surface which can direct the observer to the origin of failure. The most common markings are wake hackle markings on pores or inclusions (also called fracture tails) which are located on the side away from the fracture origin (Fig. 3). These wake hackle marking usually fan out away from the fracture origin due to crack propagation. Wallner lines are interference patterns of the propagating crack with stress waves and are approximately perpendicular to the general direction of propagation. Twist hackle markings (also called river marks) are other features that are often observed on the fracture surface of ceramics. These markings occur due to a slight local shear stress or structural inhomogeneity in addition to the main tensile stress controlling the overall direction of crack propagation. The fracture origin is observed at the approximate center of all these markings. Once the location of the origin is found, quantitative measurements of the critical crack size can be made using an optical microscope or SEM. We determined the fracture origins by examining the fracture surface and tracing the characteristic markings back to the initiation site [12]. These markings included twist hackle (river marks), wake hackle (fracture tails), Wallner lines, and branching locations. SEM (JSM 5310, JEOL, Tokyo, Japan) images
of the fracture surfaces were taken and the fracture origin regions were magnified to characterize fracture patterns. The general fractographic procedure is outlined in ASTM standard C1322 [20]. We measured crack-initiating flaws to determine the failure stress of each specimen. Most mechanically induced cracks can be idealized as semi-elliptical and its dimension can be characterized using crack depth, a, and half-width, b. The crack sizes are approximated by an equivalent semicircular crack size, c [c = (ab)1/2 ] (Fig. 4). Stress at failure, f , can be calculated based on the known fracture toughness, KC , and the crack size, c: f =
KC √ Y c
(1)
where Y is a geometric factor, which accounts for the shape of the fracture-initiation crack and loading condition and also depends on the ratio a/b [21]. The equation [c = (ab)1/2 ] allows many irregular crack shapes to be analyzed, and avoids the complications of calculating a geometric factor for each crack [22]. For surface cracks that are small relative to the thickness of the sample, Y ∼ 1.24. For sharp cracks that are induced by a Vickers or Knoop indentation, because of the influ-
Fig. 3 – Fracture surface of a clinically failed Y-TZP-based fixed partial denture. Wake hackle markings were used to establish the reference points for determining the fracture origin in the glass veneer. Wake hackle markings were also observed on pores as an outcome of fracture passage around the pores. The markings indicate the direction of the fracture origin in the veneer layer.
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Fig. 4 – Typical fracture surface features for brittle materials depicted by a line diagram (regions not drawn to scale). Strength or sample dimensions have to be great enough to observe all regions. Often, only the mirror region is observed, as for the veneer fracture surface in clinically failed FPDs.
ence of local residual stress, Y ∼ 1.65, and for corner cracks, Y ∼ 1.4 [22]. Four-point flexure specimens from Y-TZP-based core (Cercon® ceramics, DeguDent GmbH, Hanau, Germany) and glass-based veneer (Cercon® Ceram S Veneering Ceramic, DeguDent GmbH, Hanau, Germany) were prepared to determine the fracture toughness (KC ) of the monolithic core and veneer ceramics. The final dimensions of beam specimens were 3 mm (height) × 4 mm (width) × 25 mm (length). In total, six specimens were used for each group for fracture toughness measurements. Thus, 12 beam specimens were used in the study. Specimens were indented on the veneer surface with a Vickers indenter (M-400, Leco, Tokyo, Japan) to produce controlled cracks. They were loaded to fracture in air at a crosshead speed of 0.5 mm/min by means of a four-point bending fixture in an universal testing machine (MTS Systems Corporation, Eden Prairie, MN). Flexure experiments were performed with a fixture having a 20-mm outer span and a 10-mm inner span. The strength ( f ) of beam specimens was calculated according to simple beam theory [23]. Fractography and fracture mechanics methods described above were also used to determine the fracture toughness (KC ) values of monolithic core and veneer specimens. The strength and crack size were measured and Eq. (1) was used to determine KC .
3.
Results
Clinically failed ceramic FPDs had less clear surface fracture markings due to their complex microstructure and multidirectional and repeated intra-oral loading. In addition, not all the clinically retrieved specimens had intact fracture surfaces to perform the fractographic analysis. The fragments containing the fracture origins in the veneers of two samples were missing. Therefore, their fracture origins could not be identified. Fracture origins of the Y-TZP-based fixed partial dentures were located on the gingival surface of the connectors (Fig. 3) in four of the five specimens. The remaining one specimen was failed due to fracture initiating from the margin of the posterior abutment. Wake hackle markings radiating out from pores were the main guiding features that led us to fracture origins (Fig. 3). In addition, twist hackle markings were
Fig. 5 – Fracture surface of a clinically failed Y-TZP-based fixed partial denture. Delaminations between glass veneer and Y-TZP core were observed in all Y-TZP-based fixed partial dentures.
often observed and were used to confirm the direction of fracture propagation. Fracture propagated toward the Y-TZP core after initiating at the veneer surface. However, the initial fracture stopped at the interface between the Y-TZP core and the glass veneer, as evidenced by the delamination and the planar differences of the veneer and core fracture surface at the interface (Figs. 3 and 5). For a bilayered ceramic composite, if the fracture initiates from one side and goes through the interface in a catastrophic manner (fast fracture), usually there is no planar difference between the fracture surfaces of two layers adjacent to the interface. However, if the bonding between the two layers is weak, or, the second layer is much tougher than the first layer (where fracture initiates), delamination may occur and the crack tends to stop at the interface, go along the interface and then reinitiate at certain stress concentration area of the second layer upon further loadings. We think this is due to the weak interfacial bonding and large fracture toughness difference between the Y-TZP core and the glass veneer. The Y-TZP core has a fracture toughness value of 6.4 MPa m1/2 and the glass veneer 0.7 MPa m1/2 [8]. A second fracture initiation site was observed within the core layer of each Y-TZP-based FPD (Figs. 3 and 5). As a result, the second fracture that initiated within the core layer controlled the ultimate failure. Based on the known fracture toughness and estimated critical crack sizes, the failure stresses of the veneers (initial fractures) were between 31 and 38 MPa and the failure stresses of the Y-TZP cores ranged from 379 to 501 MPa. Table 1 lists all the measured data.
4.
Discussion
Fracture surface analyses of failed ceramic fixed partial dentures showed that failure origins occurred mostly at surface flaws. Previous investigations have reported that fracture initiated typically along the veneer/core interface of ceramic crowns [24,25]. However, this was not the case in this study. In Y-TZP-based FPDs, all the connector failures were initiated from the gingival surface where tensile stresses were
d e n t a l m a t e r i a l s 2 4 ( 2 0 0 8 ) 1077–1082
the greatest due to occlusal loading. In contrast, in a previous study [12] that reported lithia-disilicate-based FPD failures from connectors, most of the connector failures were associated with fractures that initiated from occlusal surfaces. Fracture initiating flaws in lithia-disilicate-based FPDs were most likely introduced as a result of repeated contact damage. During the mastication process, the mandible makes lateral, centric, and protrusive movements that allow the opposing cusp tip in the maxilla to exert multidirectional forces on the prosthesis. As a result, fracture can occur in the most vulnerable part of the ceramic fixed partial dentures, i.e., the connector [10–13]. In addition, mechanical damage resulting from the occlusal adjustment by the dentist or the dental technician can introduce flaws in the fixed partial dentures. Fracture toughness of the glass veneer was used to calculate the stress at failure of specimens because all the fracture origins occurred within the veneer layer (Fig. 3). In this study, we found that the crack propagation of the Y-TZP core FPDs was different from that of lithia-disilicate-based FPDs. In lithia-disilicate-based FPDs the fracture-initiating crack propagates immediately upon reaching the failure stress. Even though the core layer is tougher than the veneer, once crack propagation begins in the veneer, the crack does not arrest or deviate out of the original propagation plane. Crack progression was not impeded by the core ceramic at the interface between the core and veneer. Unlike lithia-disilicate-based FPDs, the fracture in Y-TZP-based FPDs initiated from the veneer and then stopped at the interface. The secondary fracture did not occur until further loadings and interfacial delamination formed between the glass veneer and zirconia core (Fig. 5). Because of the relatively weak interfacial bonding and the much greater fracture toughness of Y-TZP, the crack stopped, turned and propagated along the interface, and then reinitiated again at the core. One can conclude that the failure occurred after the veneer fracture initiated. However, fractographic analyses showed that even though there was a fracture in the veneer layer, a FPD was still one piece until a second fracture occurred in the zirconia core. Interfacial delamination in the glass veneer/zirconia core dental ceramic structures controlled the fracture initiation sites and failure stresses of the zirconia core. The design and dimension of the connectors as well as span size of the fixed partial denture can be the key factors in causing fractures at relatively low occlusal loads but high fracture stresses. The fracture stresses of the veneer and Y-TZP estimated in this study are much less than have been reported for nonfatigued glass and zirconia [8]. This is not surprising since the clinical failed specimens were in an oral environment and the stress concentration caused by the denture design both likely adversely affected the strength of the materials. The bilayer materials also include a term in the equation for the compressive residual stress generated by the thermal expansion anisotropy and visco-elastic process [26]. Residual stresses can be determined by calculating the mirror-to-flaw size ratio and by observing the mirror boundary shape on the fracture surface [22]. However due to low toughness of the glass veneer (0.7 MPa m1/2 ) and small size of the FPD specimens, mirror boundaries were not discernable. Consequently, residual stresses could not be determined using fractographic analyses.
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The measured fracture toughness value of Y-TZP core ceramic was lower (6.4 MPa m1/2 ) in this study compared to the ones that are reported in previous literature (9–10 MPa m1/2 ) [4,5]. This is most likely due to the measurement technique in previous studies that uses the size of the indentation cracks to calculate fracture toughness. Phase transformation toughening mechanism at the tip of the indentation cracks tends to suppress crack extension. Therefore, the direct measurement technique using indentation induced cracks in zirconia ceramics may lead to a fracture toughness value greater than the actual fracture toughness. Unlike lithia-disilicate-based FPDs, fracture initiation occurred twice in Y-TZP-based FPDs due to interfacial delamination between the glass veneer and zirconia core (Fig. 5). One can conclude that the failure occurred after veneer fracture. However, fractographic analyses showed that even though there was a fracture in the veneer layer, a FPD was still one piece till fracture occurred in the zirconia core.
5.
Conclusions
Interfacial delamination in glass veneer/zirconia core dental ceramic structures controlled the fracture initiation sites and failure stresses of zirconia core. The design and dimension of the connectors as well as span size of the fixed partial denture can be the key factors in causing fractures at relatively low occlusal loads but high fracture stresses.
Acknowledgements Authors greatly appreciate the comments and help of Drs. J.B. Quinn, S.S. Scherrer, and A.D. Gemalmaz.
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