The cover photo describes the scheme of the twin screw extruder for the preparation of thermoplastic starch (TPS). As discussed by M. Paula Guarás, this screw extrusion process is very advantageous for the preparation of TPS films with improved mechanical and water vapor absorption properties in any industrial processing line for plastic packaging products.
Fabrication of Y2O3 coatings by cold-spray
Lingyan Kong1*, Rifei Han1, 2, Yang Yang1, Jiayi LI1, 2, Tianying Xiong1, Tiefan Li1
1Institute of Metal Research, Chinese Academy of Sciences
2School of Materials Science and Engineering, University of Science and Technology of China
Adv. Mater. Lett., 2019, 10 (3), pp 189-192
DOI: 10.5185/amlett.2019.2188
Publication Date (Web): Dec 31, 2018
Copyright © 2019 VBRI Press
E-mail: lykong@imr.ac.cn
As a candidate material against plasma etching, yttrium oxide has been coated onto etching chamber by plasma spray technique. However, the plasma spray technique introduces undesirable coating properties such as porous structure and deleterious thermal effects. To reduce the disadvantage of thermal impact, cold spray was used as an alternative technology to deposit thick and dense yttrium oxide coatings. Many studies have shown that the powders suitable for cold spray process should be with a size around 20μm. However, the Y2O3 are ceramics, it is difficult to form coatings by cold spray due to the lack of ductility when using the powders with a size about 20 μm. It is also difficult for nano-particles to get through the bow shock of cold spray process, which may cause deceleration or even deflection of lighter particles away from the surface, and therefore fail to be cold sprayed. In this paper, we use Y2O3 with an original average size of 30 nm to form agglomerated Y2O3 particles by hydrothermal treatment. After the hydrothermal treatment, the nano-size Y2O3 agglomerated together to a size around 20 μm and then deposited by cold spray. Y2O3 coatings were forming in this way. In this research, Y2O3 nanoscale powder was tailored into a loose agglomerated structure by hydrothermal treatment, and it was found that the addition of inorganic salt promote the agglomeration process. Cold spray experiments verified the cold spray suitability of the as-modified particle. Gas temperature greatly affects the coating thickness and microstructure, and optimal spraying parameter was fixed at 600 °C. An excellent yttria coating was successfully fabricated on aluminum alloy 6061 with a maximum thickness of 200 µm and a low porosity less than 1% using compressed air as propellant gas. The loose aggregated feedstock fractured during impact instead of deformation. It is demonstrated that particle structure is key factor for ceramic deposition by cold spray technique.
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