Journal of Dental Medicine

Background and Aims: The present investigation was conducted on the evaluation of the microstructure, phase transformation behavior, mechanical properties, and fatigue resistance of nickel-titanium rotary endodontic files. The main purpose of this investigation was to find the effect of microstructural parameters on the performance of these files.
Materials and Methods: Three rotary tools with brand names Denco, Edgeendo, and Eighteeth were studied. To evaluate the microstructure and transformation behavior of tools, metallography, X-ray diffraction, and differential scanning calorimetry methods were used. The bending test was used to check the behavior of superelasticity and memory and the microhardness test was used to evaluate the hardness of the samples. Also, cyclic fatigue behavior in a mold containing a simulated tooth canal was investigated and calculated by parametric statistical analysis. The fracture surface of the samples was also studied by scanning electron microscope.
Results: Analyses revealed the existence of a high volume fraction of R phase in the microstructure of the Edgeendo file. Rotary files with higher volume fraction of martensitic phase showed higher shape memory effect and fatigue resistance. Denco and Edgeendo rotary files with better surface quality and high volume fraction of martensitic phases in the microstructure showed better fatigue resistance compared with the Eighteeth file. In all the results, the P-value was less than 0.05.
Conclusion: Multiphase microstructure containing austenite and martensite together with appropriate phase transformation and hardness of the alloy have key roles on functional properties of rotary files. This study provides dentists with a deeper understanding of the fatigue behavior and mechanical properties of these files, enabling them to make more informed decisions in choosing the appropriate instruments for endodontic treatment.

Background and Aims: It is very important and practical to make machinable blocks with the ability to maintain the edge and at the same time have sufficient hardness for chewing. One of the most important groups of materials for making dental crowns and veneers for dentistry by CAD-CAM is lithium disilicate ceramics such that extensive research is being conducted in the field of modifying their structure and properties. All-ceramic restorations enhance the beauty of teeth due to their closeness to the tooth color and transparency. At the same time, these materials also have high biocompatibility. The main goal of this study was to evaluate the effect of hot-pressing temperature on the microstructure and mechanical properties of the lithium disilicate block fabrication.
Materials and Methods: The present laboratory study was conducted on the lithium disilicate blocks produced by powder metallurgy at Islamic Azad University, Khomeini Shahr Branch. In this regard, initially, the raw materials (including: Tetraethyl orthosilicate, Lithium nitrate, Triethyl phosphate, potassium nitrate) were processed by sol-gel method and the resulting frit (the glassy state of a ceramic) was crushed by a ceramic mortar. Then, the 3 groups of samples (n=3) were subjected to the hot-pressing at different temperatures of 750 and 900 °C for 60 minutes at a constant pressure of 200 kPa. Subsequently, the microstructure and elemental phases of lithium silicate samples were examined using scanning electron microscopy (SEM) and X-ray diffraction (XRD). Statistical analysis of the results was performed using one-way analysis of variance (ANOVA). A p-value of less than 0.05 was considered significant.
Results: X-ray diffraction results on the resulting powder before hot pressing showed the formation of a glass phase with lithium metasilicate and after hot pressing, the formation of a lithium disilicate phase. The results also showed that in all samples, the lithium disilicate phase was formed and had some porosity. In addition, with increasing the pressing temperature, the size of lithium disilicate particles increased. However, with increasing the hot-pressing temperature from 750 to 900°C, the amount of porosity decreased from about 23% to 20%.
Conclusion: The hot-pressing temperature had a significant effect on the microstructure and phases present in the produced ferrite sample. It was concluded that by changing the hot-pressing temperature, the microstructure and consequently the properties of the lithium disilicate blocks can be modified to achieve improved properties for dentistry applications.