A highly strain and damage‐tolerant thermal barrier coating fabricated by electro‐sprayed zirconia hollow spheres

Abstract

An approach to make air plasma sprayed (APS) thermal barrier coatings (TBCs) with the enhanced strain and damage tolerance was reported, using a novel hollow spheres produced by electro‐spraying (ESP) technique. Compared with agglomerated & sintered (A&S) and hollow spherical (HOSP) yttria‐stabilized zirconia (YSZ) powders, the ESP powder showed a unique network microstructure and the TBCs exhibited a 2‐3 times longer thermal cycling lifetime. The splat morphology and the top coats microstructure were investigated. Some semi‐melted ESP particles were observed in the as‐sprayed top coat. The indentation coupled with the Raman mapping technique was employed to evaluate the strain and damage tolerance of the TBCs. The coatings deposited by the ESP powder show a lower in‐plane stiffness determined by three‐point bending tests. It is proposed that the superior performance is attributed to the lower amount of the short microcracks (0.5‐4 μm) with low angle (<45°) and the semi‐melted ESP particles remained in the YSZ top coat.

Sintering behavior of a nanostructured thermal barrier coating deposited using electro‐sprayed particles

“…Due to the ferroelastic toughening mechanism, t′ zirconia have a higher toughness than c zirconia, making it become the most popular material for the top coat of TBCs 4,5 . However, our previous experiment exhibited that ESP coatings had a longer lifetime than HOSP coatings, suggesting that the phase composition had no negative impact on life time for ESP coatings 34 . One of the possible reasons was that the difference in the t’ phase content diminished during the heat treatment, especially at 1400°C, as shown in Figure 5.…”

“…Moreover, the internal stress and the connected microcracks of length similar to the grain size accompanied by phase decomposition are the dominant factors which deteriorate the mechanical properties of TBCs 50 . From this perspective, on one hand the ESP coatings exhibited an enhanced strain and damage tolerance benefited from more porous nanozones 34 . On the other hand, the t‐m transformation with 5%–7% volume expansion would be more harmful to the stability of TBCs, which occurred earlier in the HOSP coatings in the same heat treatment (Figure 6).…”

“…The slurry was firstly prepared by mixing the milled 8YSZ powder, PES and NMP with the weight ratio of 3:1:6. An optimized voltage of 20 kV and slurry flow rate of 2.5 mL/min were selected in the electro‐spraying process in order to fabricate feedstock powder with appropriate flowability and particle size (35‐85 μm) based on previous works 34,35 . Finally, the ESP feedstock powder was obtained after the sintering at 1200℃ for 2 hours in a muffle furnace to completely remove the organic materials (mainly PES) and achieve robust structures.…”

“…APS TBC top coat made from electro-spraying particles (ESP) feedstock powders exhibited the characteristics as nanostructured coatings. 34,35 The previous research have suggested that the ESP coatings had a 2-3 times longer thermal cycling lifetime than conventional HOSP coatings due to their enhanced strain and damage tolerance. 34 As a continuing study, this work is related with the sintering behavior of the coatings deposited using ESP powder, particularly in the evolution of microstructure, phase and thermal conductivity during longterm heat treatment.…”

Hollow ceramic microspheres prepared by combining electro-spraying with non-solvent induced phase separation method: A promising feedstock for thermal barrier coatings

“…Therefore, compared with the HOSP coating, the ESNP coating has a higher porosity, thus may achieve lower thermal conductivity. The increased population of micrometer-sized pores could also improve the strain tolerance and life time of the as-sprayed coating reported in our unpublished results [31]. According to the critical coordination number theory [32][33][34], if a pore is surrounded by too many grains (usually >10) [33], then the free energy gain due to the extension of the grain-boundaries into the pore outbalances the free energy loss due to the elimination of the pore surface, thus pores will be stable in the structure and there will be a limiting density even though removal of the pores would lower the energy of the system.…”