Zirconia psz why is it hard and fracture tough

The strength of Hz seems to reduce after receiving the surface modification due to containing a lower t-phase. The phase transformation of Hz can occur in form of either t- or c- to rhombohedral r- phase. The r- or distorted t-phase was found in zirconia after receiving surface modification for example sandblasting, machining, ground with a diamond bur, and polished procedure 10 , It was observed at all levels of grinding and for various amounts of yttria dopant, even in the fully stabilized c-zirconia The relationship of the r-phase on strength has never been reported.

However, the volume change of approximately 3. Fracture toughness is a mechanical characteristic of brittle material to resist crack propagation under applied stress. It is measured by the amount of energy required for fracture which is quantified by the stress-intensity factor KIC 4 , The crack lengths are in an inverse proportion to the toughness of the material 15 , The indentation load was different for each material, so, the optimum load should verify and beforehand 16 , Since the post-sintering processes of monolithic zirconia are unavoidable procedures.

There are few studies about the effect of the post-sintering processes on fracture toughness of zirconia There is no study till now related to fracture toughness of Hz.

There is no standard protocol for monolithic zirconia adjustment after sintering. The controversy exists regarding the effects of clinical adjustment by grinding with burs, finishing, polishing, glazing, or heat treatment on fracture resistance. Hence, the purpose of this study was to investigate fracture toughness of Cz and Hz after receiving different post-sintering processes.

The null hypothesis was that glazing, finishing and polishing, overglazing after polishing, and heat treatment after polishing of either classical or high translucent monolithic zirconia would not affect differences in fracture toughness. The zirconia discs were compensated for sintering shrinkage with the enlargement factor of 1.

One hundred and twenty zirconia discs of thickness 1. The mixture of glazing paste and liquid IPS e. All specimens were randomly divided into four groups according to post-sintering surface treatment: AG as-glazed , FP finished and polished , FPOG finished, polished, and overglazed , and FPHT finished, polished, and heat-treated groups. The specimens in the AG group did not receive any surface treatment.

The contact pressure was exactly 50 grams, and the finishing time for each step was 30 seconds in a continuous stroke. The horizontal movement was conducted in one direction with the custom-made load and direction-controlled machine with a fixture for holding the grinding handpiece Fig. The bur was changed to a new one for every single specimen. The specimens in the FPOG group were ground, finished, and polished similar to those in the FP group and then ultrasonic cleaned, steam cleaned, and finally overglazed, as previously described.

Custom-made machine A was used for controlling force f and direction d during finishing and polishing on the surface of zirconia z with bur b in the fixture mounted hand-piece h.

Five indentations were performed at equidistance between the indentation point and the center of each disc, and the crack length was measured and used to calculate fracture toughness D. Five indentations were performed at equidistance between the indentation point and the center of each disc.

The crack length was measured [Figure 1 D ] by using the optical microscope Olympus, Tokyo, Japan , and used to calculate fracture toughness KIC by equations 1 and 2 The mean and standard deviation sd of KIC for each group of monolithic zirconia were calculated, compared, and then further analyzed using ANOVA in conjunction with a post hoc Bonferroni multiple comparisons using statistical software SPSS version 22, Chicago, IL to determine significant differences in the KIC of monolithic zirconia materials with different post-sintering processes.

The specimens were coated with gold at a current of 10 mA and a vacuum of m-torr for three minutes, then dried in a desiccator, and finally evaluated the microstructures with a scanning electron microscope SEM, Hitachi SN, Osaka, Japan. In addition, all groups with the non-glazed surface was a significantly higher KIC than other groups with glazed-surface. Both Cz and Hz zirconia demonstrated a large amount of t- and c-phase, with no m-phase existed.

The XRD patterns of Cz mostly indicated the t- phase and a minor amount of the c- phase, vis versa for Hz. The relative phase concentration was shown in Table 1. SEM photomicrographs of topographic surfaces A-H 3. This study indicated that post-sintering processes significantly affected fracture toughness of different types of monolithic zirconia.

Therefore, the null hypothesis was rejected for the post-sintering processes, types of zirconia, and their interactions. Fracture toughness is the characterizing value of stress absorption in the material at the crack site before the catastrophic failure occurs 14 , The IF method is a simple and non-destructive technique that frequently uses when the material possesses high hardness or high strength The selection of proper load is crucial to prevent forming crack branches and chipping of the material surface 17 , Post sintering adjustment of restoration through grinding, finishing, and polishing procedures is a stepwise method, which is necessary to proceed from the coarsest grit to the finest grit size.

These procedures are needed to achieve a smooth, mirror-like surface that provides less susceptibility to bacterial plaque accumulation and minimizes deleterious effects of low-temperature degradation LTD and wears of antagonist natural dentition 21 , 23 - Such surface adjustment is unavoidable even if the restoration is close to perfect after milling and sintering.

The restoration must be adjusted clinically during the trial process, before cementation 22 , The ground zirconia showed significant deterioration in its long-term, which is negatively affected by aging 5. Some studies claim that grinding by coarse diamond burs improves the strength because of the transformation toughening mechanism 25 , Some studies have found no significant correlation between roughness and strength, especially when using a small diamond grit size 5 , 23 , However, the r-phase was eradicated after heat treatment in this study, as supported by other studies 10 , This indicates that the occurrence of the r-phase leads to a fracture toughness enhancement for FP groups.

The sequential multistep polishing procedures are recommended and widely used because of their ability to produce high-gloss surfaces in zirconia comparable to glazed surfaces. The gloss finishing was also produced by applying glaze material, but the fracture toughness results were significantly lowered, possibly because of moisture in the glazing mixture and heat from the glaze firing It was found that the mixture of glazing components trapped air bubbles within the glazed layer.

The air bubbles inside the glazed layer may represent a trigger point of failure. Moreover, the glass matrix in the mixed glazing paste did not properly melt or adhere to the zirconia, as it does with glass-based ceramics. Nevertheless, in areas demanding high esthetics, glazing shall be applied to the zirconia because the polishing procedure can decrease its brightness and produce disharmonious color compared to the natural teeth The heat treatment process seems to be less affected with Hz, probably because of the lower ability of the Hz to change phase.

This result was consistent with that of other studies 20 , The SEM showed the surface irregularities of the FPHT which did not differ from those of the FP, which means that the heat treatment did not repair the flaws or porosity of the surface. Thus, the LTD or aging of zirconia can occur and may weaken the restoration in the long term. Aging may be reduced by heat treatment, which is helpful for the long-term service, as found in another study The glazed surface consisted of low-fusing fluorapatite glass-ceramics that cannot withstand the load as much as the zirconia surface does.

The 1 kg load on the glazed surface was capable of producing crack length, which similar to the glass-based dental ceramics While 10 kgs load was needed for the polished surface to produce crack length, but quite shorter than the glazed surface. Interestingly, the finishing and polishing procedure FP groups showed the higher fracture toughness, while the glazed zirconia showed lower fracture toughness FPOG and AG groups for both Cz and Hz. Moreover, there is some area around the indented area of the glazed surface that could be the delamination of the glazed layer from the zirconia underneath.

Upon impact, the tetragonal precipitates undergo a stress induced phase transformation near an advancing crack tip. This action expands the structure as it absorbs a great deal of energy, and is the cause of the high toughness of this material.

The amount of tetragonal can be controlled by additions of the blends above to balance toughness against loss of strength. Cubic Zirconia: This single crystal optically clear form of ZrO 2 has a relatively low fracture toughness and strength, but very high thermal shock resistance.

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