Influence of Multiple Firing on the Bending Strength of Zirconia/Porcelain Bilayered Dental Ceramics

Bilayered three-point bending strength specimen of KAVO zirconia-Vita VM9 veneering porcelain were made according to ISO6872 standard, the core to porcelain thickness ratio was 1:1. Specimens were subjected to multiple dentine porcelain firing cycles. Three-point bending strength were tested, SEM was used to observe the characteristics of the failure mode. Results show that there was no influence on the bending strength by multiple firing, the three-point bending strength were 996 ~ 1020 MPa after multiple firing, and no obvious defects was found at the interfaces. In all bilayered specimen, the bulk fracture initiated at the tensile surface of the core material and was consistently accompanied with porcelain crush and delamination at the interface.

Nano DCPA-Whisker Composites with High Strength and Ca and PO4 Release

The main challenges facing composite restorations are secondary caries and bulk fracture. The objective of this study was to develop nano DCPA (dicalcium phosphate anhydrous)-whisker composites with high strength and Ca and PO4 ion release to combat caries. Flexural strength for the nano DCPA-whisker composites at a nano DCPA:whisker mass ratio of 1:2 ranged from (148 ± 9) MPa to (167 ± 23) MPa, significantly higher than the (103 ± 32) MPa of an inlay/onlay commercial control composite without Ca-PO4 release. The nano DCPA-whisker composite released PO4 to a concentration of (1.95 ± 0.13) mmol/L and Ca of (0.68 ± 0.05) mmol/L. Compared with previous conventional Ca- and PO4-releasing composites, the nano DCPA-whisker composites had strengths two-fold higher, and released comparable or higher levels of Ca and PO4. In conclusion, combining nano-DCPA with whiskers yielded novel composites that released high levels of Ca and PO4 requisite for remineralization. These high-strength composites may provide a unique combination of stress-bearing and caries-inhibiting capabilities.

Atomistic Studies of Crack Branching at Bimaterial Interfaces: Preliminary Results

ABSTRACTIn this paper, we summarize recent progress in applying atomistic studies of cracking along interfaces of dissimilar materials under quasi-static crack growth conditions. We consider two linear-elastic material strips in which atoms interact with harmonic potentials, with a different spring constant in each layer leading to a soft and a stiff strip. The two strips are bound together with a tunable potential, which allows to independently control the interface and bulk fracture surface energy. An initial crack serves as initiation point for the failure. This provides a model system to investigate how elastic properties and interface strength interplay and determine the crack growth direction, leading to either interfacial cracking or branching into the film material. We observe a clear transition to interface failure when the interface fracture energy is less than 80% of the bulk fracture energy. We further find that branching in the film material is controlled by the elastic properties of the film material, suggesting interfacial cracking for extremely soft films and branching for stiffer films. Analysis of the virial stress field around the crack suggests that the circumferential hoop stress controls the branching behavior.

Cryo-Fracturing and Cryo-Planing for In-Lens Cryo-SEM, Using a Newly Designed Diamond Knife

High-pressure frozen rat pancreas tissue samples were cryo-fractured and cryo-sectioned with a diamond knife in the microtome of a freeze-etching device. The bulk fracture faces and block faces were investigated in the frozen-hydrated state by use of a cryo-stage in an in-lens SEM. With this combination, relevant biological structures can be investigated with a few nanometers resolution in a near lifelike state, preventing the artifacts of conventional fixation techniques. Compared to TEM replica techniques, the presented method is more versatile since no replica cleaning is necessary. Additional structures can be made visible by controlled sublimation of ice, leading to a better understanding of the three-dimensional organization of organelles, such as the endoplasmic reticulum.