ZrB2-SiC Composites with Rare-Earth Oxide Additives

The effect of different content (2, 5, and 10 wt.%) of two different types of rare earth (RE) oxides (Eu2O3 and Lu2O3) on the sintering, microstructure, room temperature mechanical properties, and ablation resistance of ZrB2-25vol.% SiC ceramics were investigated. The materials were prepared using non-reactive Field Assisted Sintering Technology (FAST) in the temperature range of 1950°C – 2050°C, with a pressure of 70 MPa and a dwell of 7 min. No significant effect of the addition of 2 and 5 wt.% RE2O3 on the microstructure and the room temperature mechanical properties (hardness, indentation fracture toughness, and flexural strength) were observed. On the other hand, the coarser microstructures led to the deterioration of flexural strength and the hardness of the composites sintered with 10 wt.% RE2O3. The ablation resistance of the materials (tested up to ~ 2670°C) gradually increased with the increasing amount of RE oxides. The material with 10 wt.% Lu2O3 showed the best ablation resistance among all of the investigated compositions, with more than two times a lower ablation rate than that of the reference ZrB2-25vol.%SiC

Properties of porous multi-layered free-standing ceramic microchannels

Preparation of porous microtubular materials has attracted growing interest in recent decades. The free-standing microchannels investigated here were prepared by the microtemplating method, which allows the preparation of a multilayered material from a single suspension. The mechanical properties were characterized using the three-point flexural test and the tension test. The results present the possibility of significantly reducing the influence of structural defects by the formation of a single ceramic multi-layered structure via repeated deposition of thin monolayers

Mikromechanisticke aspekty iniciace krehkeho lomu.

Available from STL Prague, CZ / NTK - National Technical LibrarySIGLECZCzech Republi

Interfacial characterisation in transparent spinel matrix reinforced by SiC fibre

The behaviour of a propagating crack when an interface is present is a key feature in composite materials. This interaction usually predetermines final fracture behaviour of the material. In case of ceramic based composites (particle, fibre reinforced, laminates etc.) knowledge about properties of the interface and/or interlayer is usually unknown due to difficulties associated with their experimental determination. The aim of this work is to indicate a possible way for the determination of interface characteristics suitable as inputs for numerical calculations. The introduced testing methodology was tested on spinel composite ceramics reinforced by unidirectional SiC fibre. The adopted method for fracture toughness determination is using a chevron notch technique. Additionally, fractographical techniques were used when interpreting the measured values

Comparison of microleakage under orthodontic brackets bonded with five different adhesive systems: in vitro study

Abstract Background Orthodontic treatment is associated with numerous adverse side effects, such as enamel discoloration, demineralization or even caries. The presence of microleakage between the enamel and the adhesive and between the adhesive and the base of the orthodontic bracket allows penetration of the bacteria, molecules, and liquids into the enamel and can lead to unpleasant “white spot lesions” or secondary caries beneath and around the brackets. The aim of this in vitro study was to evaluate microleakage in five adhesive systems commonly used in orthodontic practice for bonding brackets. Methods One hundred extracted premolars were divided into five groups of twenty teeth. Stainless steel Legend medium metal brackets were bonded to teeth using five adhesive systems: resin-reinforced glass ionomer cement GC Fuji Ortho LC (GCF) and composite materials Light Bond (LB), Transbond XT (TB), Trulock™ Light Activated Adhesive (TL), and GC Ortho Connect (GCO). The specimens were subjected to thermal cycling, stained with 2% methylene blue, sectioned with low–speed diamond saw Isomet and evaluated under a digital microscope. Microleakage was detected at the enamel-adhesive and adhesive-bracket interfaces from occlusal and gingival margins. Statistical analysis was performed using generalized linear mixed models with beta error distribution. Results Microleakage was observed in all materials, with GCF showing the highest amount of microleakage. Composite materials GCO, TB, and LB exhibited the lowest amount of microleakage with no statistical difference between them, while TL showed a statistically significantly higher amount of microleakage (p < 0.001). The enamel–adhesive interface had more microleakage in all composite materials (GCO, LB, TB, and TL) than the adhesive bracket–interface (p < 0.001). The highest amount of microleakage occurred in the gingival region in all materials. Conclusion Composite materials showed better adhesive properties than a resin-reinforced glass ionomer cement. The presence of microleakage at the enamel-adhesive interface facilitates the penetration of various substances into enamel surfaces, causing enamel demineralization and the development of dental caries