In situ characterization of high-temperature mechanical behaviors of freestanding Si, BSAS and Yb2SiO5 environmental barrier coatings by three-point bending tests based on digital image correlation

The high-temperature mechanical properties and failure mechanisms of Si, BSAS and Yb2SiO5 ceramic materials are important for environmental barrier coatings. The Si coating, BSAS coating and Yb2SiO5 coating were prepared by the atmospheric plasma spray (APS) technique. The elastic modulus, fracture strength and fracture strain are determined by three-point bending combined with the digital image correlation (DIC) method. The results show that as the test temperature increases from 25 °C to 1000 °C, the elastic modulus of the Si coating and Yb2SiO5 coating decreases while the elastic modulus of the BSAS coating increases. And the fracture strength and fracture strain of the Si coating, BSAS coating and Yb2SiO5 coating increase. The elastic modulus is related to the interatomic distance and bonding strength of atoms. The fracture strength and fracture strain increase with the densification of the coating

Analysis of Formation Mechanisms of Sugar-Derived Dense Carbons via Hydrogel Carbonization Method

Four kinds of sugar (glucose, fructose, sucrose, and maltose) were selected as carbon precursors, and corresponding dense carbon products were prepared using a novel hydrogel carbonization method. The carbonization processes of sugar–polyacrylamide (sugar–PAM) hydrogels were studied in detail. The molecular structures in the raw materials were analyzed by proton nuclear magnetic resonance spectroscopy (1H NMR). Samples prepared at different temperatures were characterized by thermogravimetry analysis (TGA) and Fourier-transform infrared (FTIR) spectroscopy. The morphology and microstructure of sugar-derived carbons were confirmed by field-emission scanning electron microscopy (FESEM) and X-ray diffraction (XRD). The results indicated that the sugar solution was surrounded by PAM with a three-dimensional network structure and formed hydrogels in the initial stage. The sugar solution was considered to be separated into nanocapsules. In each nanocapsule, sugar molecules could be limited within the hydrogel via walls formed by PAM chains. The hydroxyl group in the sugar molecules connected with PAM by the hydrogen bond and intermolecular force, which can strengthen the entire hydrogel system. The self-generated pressure of hydrogel constrains the foam of sugar during the heat treatment. Finally, dense carbon materials with low graphitization instead of porous structure were prepared at 1200 °C

Effects of Ag and Co microalloying on glass-forming abilities and plasticity of Cu-Zr-Al based bulk metallic glasses

The influences of Ag and Co additions on the thermal stability and glass-forming abilities of twenty (Cu46-xZr47Al7Agx)100-yCoy (x = 0, 1, 2, 3, 4 and y = 0, 0.5, 1, 1.5) alloys were systematically investigated by X-ray diffraction and differential scanning calorimeter techniques. The mechanical performances (i.e., hardness, strength, and plasticity) of the alloys were evaluated, and the plasticization mechanisms of four representative (Cu46-xZr47Al7Agx)100-yCoy specimens were discussed based on the observations of scanning electron microscopy and high-resolution transmission electron microscopy. It is found that (Cu43Zr47Al7Ag3)99.5Co0.5 bulk metallic glass exhibits excellent thermal stability and improved plasticity after compression tests. The improvement can be attributed to the large amount and density of shear bands, the coexistence of a large area of smooth regions, the vein-like pattern, and the river-like pattern on the fractures. More importantly, the new-formed B2-CuZr phases and nanocrystallines during compressive tests also contribute to the improved plastic deformation ability by exalting the number and density of shear bands