Mechanism of Competitive Grain Growth in 8YSZ Splats Deposited by Plasma Spraying
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Zhenhuan Zheng, Jianxia Luo, and Qiang Li (Submitted December 23, 2014; in revised form February 27, 2015) The competitive growth mechanism of columnar grains generated from rapid solidification in plasmasprayed yttria-stabilized zirconia (YSZ) splats collected on stainless steel at different temperatures was studied. The microstructure of YSZ splats was examined quantitatively by scanning electron microscope and x-ray diffraction. The crystalline orientation of columnar grains was investigated by electron backscattered diffraction. It was found that the competition between the columnar grains was governed by their relative growing rate. The columnar grains with growing face (001) had the slowest growing rate, while those with growing face {011} had the fastest growing rate. The degree of competition between the columnar grains was related to the nucleation rate for the splat at different substrate temperatures (Ts). When the splat was at Ts = 300 K or Ts = 373 K, the relatively low nucleation rate resulted in less competition between the columnar grains; thus, no preferential orientation was observed. In contrast, for the splat at Ts = 473 K, the higher nucleation rate resulted in more competition between the columnar grains, increasing the proportion of grains with the faster growing face {011}, and therefore, the columnar grains show Æ011i preferential orientation.
Keywords
competitive grain growth, plasma spraying, splat, substrate temperature, yttria-stabilized zirconia
1. Introduction Zirconia stabilized with 8 wt.% yttria (8YSZ) has been established as a standard thermal barrier coating (TBC) material because of its low thermal conductivity and excellent thermal shock resistance (Ref 1). Atmospheric plasma spraying (APS) is widely used to fabricate 8YSZ coatings due to its high deposition rate and cost effectiveness (Ref 2, 3). In the APS process, powder particles are melted and accelerated in a plasma jet, and then impinge upon the substrate, where they rapidly spread and solidify. The solidified lamellae are known as splats and are the building blocks of the whole coating. Therefore, the microstructure and properties of YSZ coatings depend on individual splat morphology, adhesion between the splat and the substrate, cohesive strength among individual splats, and the microstructure of the splats themselves (Ref 4-6). The measured cooling rate of the YSZ splat sprayed on a stainless steel substrate is greater than 106 K/s (Ref 7). Such rapid cooling results in copious nucleation followed by the growth of columnar grains within splats because of unidirectional solidification (Ref 8). A number of Zhenhuan Zheng, Jianxia Luo, and Qiang Li, School of Materials Science and Engineering, Fuzhou University, Fuzhou 350116, China. Contact e-mail: [email protected].
Journal of Thermal Spray Technology
theoretical and experimental studies have been conducted in order to better understand the mechanisms of rapid solidification and microstructure development in plasmasprayed YSZ splats (Ref 9-13). Wang et al. (Ref 9) de
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