Residual stresses in bilayer dental ceramics
ABSTRACT It is clinically observed that lithia-disilicate-based all-ceramic fixed partial dentures (FPD) can fail because of the fragmentation of the veneering material. The hypothesis of this study is that the global residual stresses within the surface of those veneered FPDs may be responsible for partial fragmentation of the veneering ceramic. Bilayer and monolithic ceramic composites were prepared using a lithia disilicate based (Li2OSiO2) glass-ceramic core and a glass veneer. A four-step fracture mechanics approach was used to analyze residual stress in bilayered all-ceramic FPDs. We found a statistically significant increase in the mean flexural strengths of bilayer specimens compared with monolithic glass specimens (p < or = 0.05). There was a statistically significant difference between the mean longitudinal and transverse indentation-induced crack sizes in bilayer specimens (p < or = 0.05), which indicates the existence of residual stress. Global residual stresses in the veneer layer, calculated using a fracture mechanics equation, were determined to be responsible for the increased strength and observed chipping, i.e., spallation in bilayer ceramic composites.
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ABSTRACT: This study investigated the adhesion between zirconia framework and four veneering ceramic (VC) materials with varying coefficients of thermal expansions (CTE). Zirconia rods (N = 40) (ICE Zirkon) (diameter: 4 mm, height: 20 mm) were milled and sintered. After firing, the zirconia rods were air-abraded and cleaned. They were randomly assigned to receive four VCs (N = 10/group), namely (a) Vita VM9 (VZ; 9–9.2 × 10−6 K−1), (b) Cerabien ZR (CZ; 9.1 × 10−6 K−1), (c) Matchmaker ZR (MM; 9.4 × 10−6 K−1), and (d) Ice Zirconia Ceramic (IZ; 9.6 × 10−6 K−1). The VCs were then fired onto zirconia rods (height: 2 mm, thickness: 2 mm) circumferentially and were thermocycled for 6000 times (5/55 °C, dwell time: 30 s). Specimens were loaded from the top of the zirconia rods (0.5 mm/min) in a universal testing machine until debonding. Shell–Nielsen bond strength values were calculated (MPa). Failure types were evaluated under SEM. The data were statistically analyzed (one-way ANOVA, Tukey’s; α = 0.05). Weibull distribution values including the Weibull modulus (m) (0.05) was calculated. The highest mean bond strength (MPa) was obtained for CZ (42.08 ± 4.08), followed by VZ (41.77 ± 4.92), MM (40.7 ± 3.64), and IZ (40.05 ± 5.78). While mean bond strength for VZ, MM, and IZ were not significantly different (p > 0.05), CZ was significantly higher than that of IZ (p m = 16.94) and the highest for MM (m = 20.16). Mainly, adhesive failures followed by mixed failures were observed. VCs with a greater mismatch of CTE with the zirconia framework exhibited similar Shell–Nielsen bond strength to those with fewer mismatches. CTE mismatch did not affect the results of CZ (9.1 × 10−6 K−1) and IZ (9.6 × 10−6 K−1).Journal of Adhesion Science and Technology 04/2015; 29(18). DOI:10.1080/01694243.2015.1046308 · 1.09 Impact Factor
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ABSTRACT: This in vitro study was designed to investigate the influence of the veneer and cyclic loading on the failure behavior of lithium disilicate glass-ceramic (LDG) crowns on maxillary first molar. Sixty-four LDG crowns were divided into 4 groups (n=16). Thirty-two monolithic crowns were fabricated from IPS e.max Press (M), and the remaining bilayered crowns using cut-back technique and conventional manual layering technique from IPS e.max Press/Ceram (B). Monolithic or bilayered crowns were subjected to single-load-to-fracture (SLF) testing using a universal testing machine, before (M1 and B1) and after exposure to sliding-contact fatigue (SCF) testing (M2 and B2), consisting of 1,200,000 mechanical cycles (Fmax=98N). Data were statistically analyzed using two-by-two factorial design ANOVA. Fractographic analysis was performed to determine the fracture modes of the failed specimens. The mean fracture load values (N±S.D.) for M1, B1, M2 and B2 were 2686±628N, 1443±327N, 2133±578N and 1464±419N, respectively. Significant differences were found between the failure loads of all groups (P<0.001), except between groups B1 and B2. Bulk fracture initiating from the occlusal surface is the primary failure mode of monolithic and veneered LDG crowns. Cracking that initiated from core-veneer interfacial defects and ultimately resulted in bulk fracture is another major failure origin of veneered all-ceramic crowns. Veneer application resulted in significantly lower fracture load values compared to monolithic LDG crowns. Cyclic loading is an accelerating factor contributing to fracture for monolithic LDG crowns but not for bilayered ones.Dental materials: official publication of the Academy of Dental Materials 12/2013; DOI:10.1016/j.dental.2013.11.001 · 4.16 Impact Factor
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ABSTRACT: The present work evaluated the thermal behavior of porcelain-metal and porcelain-zirconia restorations during fast and slow firing and cooling. All-ceramic (porcelain on zirconia) and porcelain-fused-to-metal (PFM) molar crowns were fabricated with 1 or 2mm porcelain thickness. Thermocouples were attached to the cementation (T1) and occlusal (T4) surfaces of the restoration and embedded at the framework-porcelain interface (T2) and inside the porcelain (T3) to acquire temperature readings by time. Slow heating was set as 45°C/min and fast heating as 140°C/min. For fast cooling, the furnace was opened immediately after the holding time. Slow cooling was effected by opening the furnace when it reached 50°C below the Tg. Porcelains Tg were calculated for each cooling rate. Slow heating rate was measured at T4 as being 30°C/min while fast heating at T4 was 100°C/min. The measured cooling rates within the porcelain (T2) around the Tg range were 20°C/min and 900°C/min for slow and fast cooling, respectively. During slow cooling, similar temperatures were found for both zirconia and metal crowns. Remarkable temperature gradients were observed for the fast cooled all-ceramic crown (T1-T4=100°C) and, of lower magnitude for PFM (T1-T4=30°C). Tg of porcelains increase with faster cooling rates. Slow cooling appears to be especially important for all-ceramic crowns to prevent high magnitude thermal gradients, which could influence cracking and fracture of the porcelain.Dental materials: official publication of the Academy of Dental Materials 09/2013; DOI:10.1016/j.dental.2013.08.212 · 4.16 Impact Factor