Highly Segmented Thermal Barrier Coatings Deposited by Suspension Plasma Spray: Effects of Spray Process on Microstructu
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Xiaolong Chen, Hiroshi Honda, Seiji Kuroda, Hiroshi Araki, Hideyuki Murakami, Makoto Watanabe, and Yoshio Sakka (Submitted June 23, 2016; in revised form September 21, 2016) Effects of the ceramic powder size used for suspension as well as several processing parameters in suspension plasma spraying of YSZ were investigated experimentally, aiming to fabricate highly segmented microstructures for thermal barrier coating (TBC) applications. Particle image velocimetry (PIV) was used to observe the atomization process and the velocity distribution of atomized droplets and ceramic particles travelling toward the substrates. The tested parameters included the secondary plasma gas (He versus H2), suspension injection flow rate, and substrate surface roughness. Results indicated that a plasma jet with a relatively higher content of He or H2 as the secondary plasma gas was critical to produce highly segmented YSZ TBCs with a crack density up to ~12 cracks/mm. The optimized suspension flow rate played an important role to realize coatings with a reduced porosity level and improved adhesion. An increased powder size and higher operation power level were beneficial for the formation of highly segmented coatings onto substrates with a wider range of surface roughness.
Keywords
segmentation cracks, suspension plasma spray, thermal barrier coating, yttria-stabilized zirconia (YSZ)
1. Introduction Ceramic thermal barrier coatings of yttria partially stabilized zirconia (YSZ) have a widespread utilization in the hot-section components of both land-based and aeroengine gas turbines to significantly increase their operation temperatures and energy efficiency. Typical TBCs employed in gas turbines are deposited either by the process of atmospheric plasma spray (APS) or by electron beam physical vapor deposition (EB-PVD). A typical APS coating exhibits a layered microstructure with a porosity level of 10-30% comprised of pores, cracks, and inter-splats interfaces, which imparts a thermal conductivity even less than ~1 W m 1 K 1, and is essential to service durability during thermal cycling and thermo-mechanical loading. Even so, APS TBCs often show shorter lifetimes compared to EB-PVD TBCs with feathery columnar grains imparting much higher strain tolerance (Ref 1-6). Combing the advantages that APS with a high efficiency and low cost always benefits the TBC industry and inter-column gaps in the EB-PVD TBCs lead to an excellent compliance, segmented TBCs with vertical Xiaolong Chen, Hiroshi Honda, Seiji Kuroda, Hiroshi Araki, Hideyuki Murakami, Makoto Watanabe, and Yoshio Sakka, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan. Contact e-mails: Xiaolong. [email protected] and [email protected].
Journal of Thermal Spray Technology
cracks running at least through half of the coatings thickness which are beneficial for stress relaxation have been developed to achieve a prolonged service lifetime (Ref 7, 8). A segmentation crack density up to 8.9 cracks/ mm was achieved by using fused and cr
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