Effect of zirconia surface treatment on zirconia/veneer interfacial toughness evaluated by fracture mechanics method

Objectives

The aim of this study was to evaluate the effect of the airborne-particle abrasion and liner application on the interfacial toughness between veneering porcelain and zirconia core by means of a fracture mechanics test.

Methods

Beam-shaped zirconia specimens were sectioned and divided into 4 groups according to different surface treatments as follows: Group C (control): no treatment; Group L: application of liner; Group A: airborne-particle abrasion with Al₂O₃ (sandblasting); and Group AL: airborne-particle abrasion and application of liner. The zirconia surfaces before and after sandblasting were observed and analyzed by SEM and white light interferometer. Specimens of each pretreated group were veneered with 3 core/veneer thickness ratios of 2:3, 1:1, and 3:2, corresponding to 3 phase angles respectively. Fracture mechanics test was performed on each specimen, the energy release rate G and phase angle ψ were calculated to characterize interfacial toughness. The experimental data were analyzed statistically using three-way ANOVA and the Tukey's HSD test. The surfaces of fractured specimens were examined by SEM and EDX.

Results

At each phase angle, the interfaces with no treatment had higher mean G values than that of other groups. All the specimens showed mixed failure mode with residual veneer or liner on the zirconia surfaces.

Conclusions

The toughness of zirconia/veneer interface with no treatment is significantly higher than that of interfaces subjected to liner application and airborne-particle abrasion.

Clinical significance

Liner application and airborne-particle abrasion seem to reduce zirconia/veneer interfacial toughness. Therefore, the two surface treatment methods should be applied with caution.     

Strength and fracture toughness of zirconia dental ceramics

Objective

The aim of the paper is to determine and discuss the correlation between the fracture toughness and the fracture stress in zirconia transforming ceramics with a small artificial crack. As an R-curve behaviour is usually present in transforming ceramics for both small and long cracks, predictions of the fracture stress can only be done with an accurate knowledge of the R-curve and crack dimensions.

氧化锆对二硅酸锂玻璃陶瓷力学性能的影响

目的:测定牙科氧化锆-二硅酸锂玻璃陶瓷复合材料(ZTCLDC)的力学性能,探讨氧化锆质量分数变化对ZTCLDC力学性能的影响及机制。方法:向制备好的二硅酸锂基础玻璃粉体中分别添加不同质量分数的ZrO(21%,2%,3%,4%),静压成型晶化热处理,每组中的半数试件进行牙科热压铸处理,分别测试热压铸前后的三点弯曲强度及断裂韧性;扫描电镜观察晶体微观结构;X射线衍射分析其晶相组成。结果:热压铸前,ZTCLDC抗弯强度和断裂韧性随ZrO2含量的增加而升高;热压铸后,ZTCLDC抗弯强度随ZrO2含量的增加而降低,ZrO2加入量为4%时,压铸后的断裂韧性最高为4.3 MPa.m1/2。结论:ZrO2对实验二硅酸锂玻璃陶瓷存在增韧效果,能够提高复合材料的断裂韧性。     

Grain size,crystalline phase and fracture toughness of the monolithic zirconia

PURPOSEThis study evaluated the relationship among translucency, crystalline phase, grain size, and fracture toughness of zirconia.MATERIALS AND METHODSFour commercial zirconia - Prettau^®Anterior^® (PA), Prettau^® (P), InCorisZI (ZI), and InCorisTZI (TZI)- were selected for this study. The bar specimens were prepared to determine fracture toughness by using chevron notched beam method with four-point bending test. The grain size was evaluated by a mean linear intercept method using a scanning electron microscope. X-ray diffraction and Rietveld refinement were performed to evaluate the amount of tetragonal and cubic phases of zirconia. Contrast ratio (CR) was measured to investigate the level of translucency.RESULTSPA had the lowest fracture toughness among other groups (P < .05). In addition, the mean fracture toughness of P was significantly less than that of ZI, but there was no difference compared with TZI. Regarding grain size measurement, PA had the largest average grain size among the groups. P obtained larger grain size than ZI and TZI (P < .05). However, there was no significant difference between ZI and TZI. Moreover, PA had the lowest CR value compared with the other groups (P < .05). This means PA was the most translucent material in this study. Rietveld refinement found that PA presented the greatest percentage of cubic phase, followed by TZI, ZI, and P, respectively.CONCLUSIONThe different approaches are used by manufacturers to fabricate various types of translucent zirconia with different levels of translucency and mechanical properties, which should be concerned for material selection for successful clinical outcome.     

Effects of zirconia addition on fracture toughness and bending strength of dental porcelains

Zirconia dispersed composite porcelains with glass and aluminous porcelain as matrix were prepared as models of dental porcelains. The bending strength and fracture toughness of the composite porcelains were examined. The bending strength and fracture toughness of composite porcelains containing 50 wt% zirconia were 20 to 80% greater than in glass alone. However, bending strength and fracture toughness decreased upon the addition of zirconia at more than 50 wt%. Moreover, in the case of aluminous porcelain as matrix, fracture toughness increased to a maximum value of 2.6 M Pa.m1/2by addition of 23 wt% zirconia, twice the toughness of glass alone. On the other hand, no increase of bending strength was observed in this case. Deflection and bowing of cracks as well as microcracking effects were related to these increases of mechanical properties in zirconia dispersed composite porcelains.     

Effects of load and loading time on fracture toughness with indentation method

For clinical application of ceramics such as porcelains that are frequently used as crown restoration materials, it is important to quantitatively evaluate and determine brittleness. This quality is expressed as a fracture toughness value, KIC, but no distinct method for its determination has yet been established. In order to standardize conditions for the determination of KIC by the indentation method, effects of indentation load and loading time on KIC of calcium phosphate crystalline ceramics (CP) were studied at various Vickers indentation loads and various loading times in CP plate-like segments. Furthermore, plate-like segments of each of CP, apatite (AP), mica-beta-spodumene (MIS) and mica (MIC) groups were subjected to experiment at various indentation loads at a fixed loading time to study the effects of indentation load on KIC in four kinds of Castable Ceramics. The results are summarized as follows: 1) The Vickers hardness degree of CP was decreased with an increase in indentation load and loading time, reaching the maximum value (499Hv) at 1kgf of indentation load and 5s of loading time. 2) The value of half of the crack length of CP was increased with an increase in indentation load and loading time, reaching a maximum (530 microns) at 20kgf of indentation load and 30s of loading time. 3) KIC of CP reached the maximum value (2.78MNm-3/2) at 5kgf of indentation load and 5s of loading time, and the minimum (1.52MNm-3/2) at 20kgf of indentation load and 30s of loading time. 4) Optimal experimental conditions for KIC of CP determined by indentation method were 5kgf or 10kgf of indentation load and 15s of loading time. 5) KIC values (MNm-3/2) determined at 5kgf of indentation load and 15s of loading time for CP, AP, MIS and MI were 2.27, 0.95, 1.82 and 1.81, respectively. 6) The course of cracks due to indentation force showed a linear pattern of intra-granular fracture. 7) The cracks were revealed to show median cracks by fractography.     

Effect of margin design on fracture load of zirconia crowns

Zirconia‐based restorations are showing an increase as the clinicians’ preferred choice at posterior sites because of the strength and esthetic properties of such restorations. However, all‐ceramic restorations fracture at higher rates than do metal‐based restorations. Margin design is one of several factors that can affect the fracture strength of all‐ceramic restorations. The aim of this study was to assess the effect of preparation and crown margin design on fracture resistance. Four groups of bilayer zirconia crowns (with 10 crowns in each group) were produced by hard‐ or soft‐machining technique, with the following four different margin designs: chamfer preparation (control); slice preparation; slice preparation with an additional cervical collar of 0.7 mm thickness; and reduced occlusal thickness (to 0.4 mm) on slice preparation with an additional cervical collar of 0.7 mm thickness. Additionally, 10 hard‐machined crowns with slice preparation were veneered and glazed with feldspathic porcelain. In total, 90 crowns were loaded centrally in the occlusal fossa until fracture. The load at fracture was higher than clinically relevant mastication loads for all preparation and margin designs. The crowns on a chamfer preparation fractured at higher loads compared with crowns on a slice preparation. An additional cervical collar increased load at fracture for hard‐machined crowns.     

Effect of preparation design on the fracture resistance of zirconia crown copings

The aim of this in vitro study was to evaluate the effects of different preparation designs on the fracture resistance of single-crown zirconia frameworks. To this end, maxillary molar dies of CrCo alloy were fabricated with five different preparation designs: shoulderless, slight and pronounced deep chamfer, beveled and non-beveled shoulder. Ten zirconia copings with a wall thickness of 0.4 mm were fabricated for each type of preparation. After cementation by glass ionomer cement, they were loaded until fracture. There were significant differences in the breaking load of the experimental groups (ANOVA, p < 0.01). The shoulder preparation had a mean breaking load of 2286 N, the shoulderless preparation 2041 N, the beveled shoulder 1722 N, the pronounced deep chamfer 1752 N, and the slight chamfer 1624 N. Based on the results of this study, a shoulder preparation is highly recommended whenever possible. Moreover, for endodontically treated teeth that are structurally compromised or which have anatomically limited areas, the slight chamfer preparation is an optimal recommendation.     

Comparison of two fracture toughness testing methods using a glass-infiltrated and a zirconia dental ceramic

PURPOSE

The objective of this study was to compare the fracture toughness (K_Ic) obtained from the single edge V-notched beam (SEVNB) and the fractographic analysis (FTA) of a glass-infiltrated and a zirconia ceramic.

MATERIALS AND METHODS

For each material, ten bar-shaped specimens were prepared for the SEVNB method (3 mm × 4 mm × 25 mm) and the FTA method (2 mm × 4 mm × 25 mm). The starter V-notch was prepared as the fracture initiating flaw for the SEVNB method. A Vickers indentation load of 49 N was used to create a controlled surface flaw on each FTA specimen. All specimens were loaded to fracture using a universal testing machine at a crosshead speed of 0.5-1 mm/min. The independent-samples t-test was used for the statistical analysis of the K_Ic values at α=0.05.

RESULTS

The mean K_Ic of zirconia ceramic obtained from SEVNB method (5.4 ± 1.6 MPa·m^1/2) was comparable to that obtained from FTA method (6.3 ± 1.6 MPa·m^1/2). The mean K_Ic of glass-infiltrated ceramic obtained from SEVNB method (4.1 ± 0.6 MPa·m^1/2) was significantly lower than that obtained from FTA method (5.1 ± 0.7 MPa·m^1/2).

CONCLUSION

The mean K_Ic of the glass-infiltrated and zirconia ceramics obtained from the SEVNB method were lower than those obtained from FTA method even they were not significantly different for the zirconia material. The differences in the K_Ic values could be a result of the differences in the characteristics of fracture initiating flaws of these two methods.     

Evaluation of loading conditions on fatigue-failed implants by fracture surface analysis

PURPOSE: The goal of this study was to determine the relationship between fracture surface morphology and applied stress level for dental abutment screws loaded in cyclic fatigue. If a correlation between fracture surface and load level can be determined, then the fracture surface analysis could be used as a tool to assess the mechanism by which a screw failed and the magnitude of the load at which it failed. MATERIALS AND METHODS: Test implants were loaded with static and cyclic forces. In the cyclic test, the load versus the number of cycles was plotted as a curve for biomechanical analysis. The fracture surfaces of the failed screws were observed and recorded using scanning electron microscopy (SEM). RESULTS: Two fracture phases, a smooth region and a rough region, were observed on the fracture surface. After identifying the boundary between the 2 regions, the smooth region ratio (SRR), the ratio of the smooth phase area to the area of the whole fracture surface, was measured using digitized SEM images. The mean SRRs were 0.60 +/- 0.03, 0.66 +/- 0.03, and 0.75 +/- 0.03 when the tested implants were subjected to dynamic loading of 60%, 55%, and 50% ultimate failure loading (UFL), respectively. Linear relationships were found between the SRR values and loading magnitude and between SSR and number of cycles. DISCUSSION: The smooth area on the fracture surface can be used to assess the load conditions and internal stress of fatigue-fractured implants. CONCLUSIONS: These results demonstrate that fracture surface analysis of fractured implants has the potential to become a useful indicator for assessing implant fracture mechanisms.     

晶化热处理对牙用玻璃陶瓷断裂韧性的影响

目的：探讨热处理过程对玻璃陶瓷断裂性能的影响。方法：按不同晶化温度条件分组，利用X射线衍射技术鉴定晶相，计算结晶率，同时采用显微压痕法测量相应条件下材料的显微硬度（Hv)及断裂韧性（KIC)。结果：该材料的主晶相为四硅酸氟云母晶体，在830℃-950℃的晶化温度范围内，随晶化温度升高，结晶率呈上升趋势，HV和KIC也不断增大，且与结晶率之间存在相关关系。结论：玻璃陶瓷的结晶过程是影响其断裂韧性的重要因素。     

Effect of veneering technique on the fracture resistance of zirconia fixed dental prostheses

Summary To compare the fracture resistance of zirconia 3‐unit posterior fixed dental prostheses (FDPs) frameworks veneered with different veneering materials and techniques before and after artificial ageing. Forty‐eight zirconia 3‐unit FDPs, representing a missing first molar, were adhesively cemented on human teeth. The zirconia frameworks were randomly distributed according to the veneering materials and techniques into three groups, each containing 16 samples: group LV (layering technique/Vintage ZR), group LZ (layering technique/ZIROX) and group PP (CAD/CAM and press‐over techniques/PressXZr). Half of each group was artificially aged through dynamic loading and thermocycling to simulate 5 years of clinical service. Afterwards, all specimens were tested for fracture resistance using compressive load. An analysis of variance (anova ) was used to assess the effect of veneering ceramic and artificial ageing on fracture resistance (P < 0·05). Except for one minor cohesive chipping in group LV1, all specimens survived artificial ageing. The mean fracture resistance values (in Newton) of different non‐aged (± s.d.)/aged (± s.d.) groups were as follows: LV0 2034 (± 401)/LV1 1625 (± 291); LZ0 2373 (± 718)/LZ1 1769 (± 136); and PP0 1959 (± 453)/PP1 1897 (± 329). Artificial ageing significantly reduced the fracture resistance in groups veneered with the layering technique (P < 0·05), whereas no significant effect was found in specimens veneered with the CAD/CAM and press‐over techniques. All tested systems have the potential to withstand occlusal forces applied in the posterior region. The combination of the CAD/CAM and press‐over techniques for the veneering process improved the overall stability after artificial ageing, relative to the layering technique.     

Effect of glaze thickness on the fracture toughness and hardness of alumina-reinforced porcelain

STATEMENT OF PROBLEM: Although porcelain is the most esthetic restorative material available, it is subject to fracture during function. Glazing reduces the size of flaws in the surface of the porcelain and increases its resistance to crack propagation, but the optimum thickness of this glazed layer has not been determined. PURPOSE: This study compared the fracture toughness (Kc ) and Vickers hardness number (VHN) of an alumina-reinforced porcelain at different thicknesses of glaze. MATERIAL AND METHODS: Disks of feldspathic porcelain reinforced with 2% aluminum oxide were prepared and glazed for 0, 30, 60, 90, and 120 seconds. Fracture toughness and Vickers microhardness were determined with a microindentation technique. Thickness of the glazed layers was measured with a SEM. RESULTS: Mean Vickers hardness number and fracture toughness values both decreased significantly with glazing times from 0 to 60 seconds, and increased significantly with glazing times from 60 to 120 seconds (P <.001, ANOVA). SEM examination revealed an increase in glazed layer thickness with increased glazing time. CONCLUSIONS: Minimal and maximum thicknesses of glaze layers on alumina-reinforced porcelain resulted in a surface that was harder and more resistant to fracture than moderate glaze thicknesses.     

Effect of processing induced particle alignment on the fracture toughness and fracture behavior of multiphase dental ceramics

ObjectiveTo investigate the processing induced particle alignment on fracture behavior of four multiphase dental ceramics (one porcelain, two glass–ceramics and a glass-infiltrated–alumina composite).MethodsDisks (Ø12 mm × 1.1 mm-thick) and bars (3 mm × 4 mm × 20 mm) of each material were processed according to manufacturer instructions, machined and polished. Fracture toughness (K_Ic) was determined by the indentation strength method using 3-point bending and biaxial flexure fixtures for the fracture of bars and disks, respectively. Microstructural and fractographic analyses were performed with scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction.ResultsThe isotropic microstructure of the porcelain and the leucite-based glass–ceramic resulted in similar fracture toughness values regardless of the specimen geometry. On the other hand, materials containing second-phase particles with high aspect ratio (lithium disilicate glass–ceramic and glass-infiltrated–alumina composite) showed lower fracture toughness for disk specimens compared to bars. For the lithium disilicate glass–ceramic disks, it was demonstrated that the occurrence of particle alignment during the heat-pressing procedure resulted in an unfavorable pattern that created weak microstructural paths during the biaxial test. For the glass-infiltrated–alumina composite, the microstructural analysis showed that the large alumina platelets tended to align their large surfaces perpendicularly to the direction of particle deposition during slip casting of green preforms.SignificanceThe fracture toughness of dental ceramics with anisotropic microstructure should be determined by means of biaxial testing, since it results in lower values.     

Effect of the adhesive layer thickness on the fracture toughness of dental adhesive resins

We investigated how the thickness of an adhesive layer between two Co-Cr alloy plates affected the mode I fracture toughness of dental adhesive resin by varying the type of resin using a double cantilever beam (DCB) test. Two typical adhesive resins (PV and SB) were used. The adhesive layers of the DCB test specimens were 20, 100 and 200 microns thick. The fracture modes of PV differed with the thickness of the adhesive layer, such as interface fracture at 20 microns thickness, and similar cohesive fracture at 100 and 200 microns thickness. In the case of SB, crack-propagating areas were observed as cohesive fractures in all test specimens with different adhesive layer thickness, and the surfaces of these areas became remarkably rougher as the thickness of the adhesive layer increased. The fracture toughness of PV was not affected by the differences in thickness between the 100 and 200 microns adhesive layers, but there was a notable decrease in fracture toughness when the adhesive layer decreased to a thickness of 20 microns. That of SB showed a tendency to increase as the adhesive layer became thicker.     

Effect of storage media and time on the fracture toughness of resin-based luting cements

Background: Resin‐based cements are brittle materials and the major shortcomings of these materials are manifested in their sensitivity to flaws and defects. Although various mechanical properties of resin luting cements have been described, few fracture toughness test data for resin‐based luting cements using the short rod design have been published. Methods: Specimens were prepared from five resin luting cements. For each material a total of 36 disc‐shaped specimens were prepared using a custom‐made mould. Specimens were randomly divided into six groups of six, immersed in two solutions: distilled water and 0.01 mol/L lactic acid at 37 °C for 24 hours, 1 month or 3 months. The specimens were loaded using a universal testing machine. The maximum load at specimen failure was recorded and the K_Ic (MPa. M 0.5) was calculated. Results: There was a relationship between material, storage solution and time (p < 0.05). Nexus 3 showed the highest K_Ic followed by Panavia F, Calibra, Smart Cem2 and seT. Conclusions: The fracture toughness of the resin luting cements was affected by both time and storage solution. Comparable fracture toughness of conventional resin cement and self‐adhesive resin cements was observed.