Composition and microstructure properties of materials are the main processing attributes that material scientists have been focusing on in a bid to bridge the gaps that exist between the two. Based on this, the scientists have been able to design composition of materials that can achieve excellent performance, a case in point, thermal barrier coatings: which have been broadly applied in aircraft and gas turbine engines. Presently, the most popular material for thermal barrier coatings is the 8% by weight yttria stabilized zirconia (8YSZ) with non-transformable tetragonal prime phase (t´).
Recently, a novel fabrication technique: plasma spraying-physical vapor deposition (PS-PVD), has been developed and employed for the fabrication of the yttria based material since it can combine the benefits of atmosphere plasma spraying and electron-beam physical vapor deposition. However, for the PS-PVD, a lower particle size for feedstocks is necessary to facilitate the vaporization of feedstocks. Currently, M6700 powders are most popular for this purpose and have a detrimental drawback in that the remnant monoclinic zirconia has damaging effects to the thermal shock resistance capability of the resultant coating. Therefore, there is need to fabricate high performance YSZ feedstocks as substitutes for the M6700 powders.
To this note, a team of researchers at Harbin Institute of Technology in China led by professor You Wang investigated the t´ phase in yttria stabilized zirconia feedstocks used for plasma spraying-physical vapor deposition. They also hoped to pioneer in the preparation of non-transformable tetragonal (t´) n-8YSZ feedstocks by the nanopowder reconstitution method and establish a good foundation for future work. Their work is now published in the research journal, Ceramics International.
The research team commenced their work by selecting a raw nanopowder of 8YSZ of specific grain size. The 8YSZ feedstocks were synthesized by the powder reconstitution method. Next, they exposed the powders to sintering at 1250 oC for a specific duration. For comparison purposes, the researchers used 7% by weight yttria stabilized zirconia agglomerated powders of specific particle sizes (M6700). The surface morphology of the resultant powders was examined and eventually characterized.
The authors observed that surface morphology of n-8YSZ feedstocks was composed of spherical or equiaxed agglomerates which satisfied the demands of particle size for PS-PVD. When compared with the control sample, the team noted that the control sample yielded poorer sphericity. Again, an X-ray diffractometer analysis of the samples revealed that the n-8YSZ feedstocks were composed of t prime tetragonal zirconia (t´).
Feifei Zhou and colleagues demonstrated the successful fabrication of n-8YSZ feedstocks for PS-PVD using the powder reconstitution technique for the first time. It has been seen that the grain size of n-8YSZ feedstocks is below 20 nm and the n-8YSZ feedstocks are composed of spherical agglomerates whose particle size is below 23 µm. Altogether, the fabricated feedstock is advantageous over the current/control sample powders used for PS-PVD. Adopting the new n-8YSZ feedstocks would present wider applications for the PS-PVD. Currently, The spherical t´-8YSZ feedstocks with nanostructure have been industrialized in Fujian Dilong Innovation Development Co., Ltd, Quanzhou, China.
Feifei Zhou, You Wang, Yaming Wang, Liang Wang, Junfeng Gou, Wenlong Chen. A promising non-transformable tetragonal YSZ nanostructured feedstocks for plasma spraying-physical vapor deposition. Ceramics International, volume 44 (2018) pages 1201–1204
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