Thermal Aging Behavior of Axial Suspension Plasma-Sprayed Yttria-Stabilized Zirconia (YSZ) Thermal Barrier Coatings

Abstract

7.5YSZ thermal barrier coatings (TBCs) were deposited onto the stainless steel substrates using axial suspension plasma spraying (ASPS). Free-standing coatings were isothermally aged in air from 1200 to 1600°C for 24 h and at 1550°C for 20 to 100 h, respectively. Thermal aging behavior such as phase composition, microstructure evolutions, grain growth, and mechanical properties for thermal-aged coatings were investigated. Results show that the as-sprayed metastable tetragonal (t 0 -ZrO 2 ) phase decomposes into equilibrium tetragonal (t-ZrO 2 ) and cubic (c-ZrO 2 ) phases during high-temperature exposures. Upon further cooling, the c-ZrO 2 may be retained or transform into another metastable tetragonal (t 00 -ZrO 2 ) phase, and tetragonal fi monoclinic phase transformation occurred after 1550°C/ 40 h aging treatment. The coating exhibits a unique structure with segmentation cracks and micro/nanosize grains, and the grains grow gradually with increasing aging temperature and time. In addition, the hardness (H) and YoungÕs modulus (E) significantly increased as a function of temperature due to healing of pores or cracks and grain growth of the coating. And a nonmonotonic variation is found in the coatings thermal aged at a constant temperature (1550°C) with prolonged time, this is a synergetic effect of coating sintering and m-ZrO 2 phase formation.

Thermal Stability of YSZ Coatings Deposited by Plasma Spray–Physical Vapor Deposition

“…The H and E values of the coating after being heat-treated at 1550 • C for 60 h reached maximum values of 11.1 ± 1.7 GPa and 188.4 ± 19.3 GPa, respectively, representing increases by approximately 66% and 97%, respectively, compared to the as-deposited coating. Similar trends of the H and E values of coatings have been reported previously; in particular, Zhao et al found that in axial suspension plasma spraying (ASPS) YSZ TBCs, the mechanical properties firstly increased significantly with sintering, and then decreased with prolonged thermal aging time, which was caused by a t-ZrO 2 → m-ZrO 2 phase transformation after thermal exposure at 1550 • C for 60 h [21]. Sintering can increase the H and E values due to thermal exposure, which has been reported for APS, ASPS and EB-PVD coatings [27][28][29].…”

“…Sintering can increase the H and E values due to thermal exposure, which has been reported for APS, ASPS and EB-PVD coatings [27][28][29]. For example, Zhao et al showed that the H and E values of ASPS TBCs increased by about 50% and 44% on the cross-section after 1300 • C/24 h exposure in air [21]. An early study on a plasma-sprayed TBC also reported that the E value of the ceramic coating increased after exposure, which was attributed to the healing of pores and grain growth and the improvement in bonding and coherence across the splat boundary during exposure at high temperature as a result of the sintering Coatings 2019, 9, 464 9 of 11 effect [30].…”

“…Hardness (H) and Young's Modulus (E) of all coatings were measured on the cross-section before and after thermal treatment through a G-200 nanoindenter (Agilent Technologies, Oak Ridge, TN, USA) equipped with a Berkovich indenter. Specific test schemes are listed in the literature [21].…”

A constitutive model for the sintering of suspension plasma‐sprayed thermal barrier coating with vertical cracks

“…As a promising solution, suspension plasma spraying (SPS) facilitates the usage of nanostructured feedstock and the implementation of strain tolerant columnar or vertically cracked microstructures . These features are believed to improve the sintering resistance and mechanical performance of SPS TBCs, comparing to conventional air‐plasma‐sprayed (APS) coatings. Nevertheless, the sintering behavior of SPS TBCs remains to be further understood.…”

Microstructure design for blended feedstock and its thermal durability in lanthanum zirconate based thermal barrier coatings

“…In addition, the poor sintering resistance of YSZ at higher temperatures reduces porosity in the top coat, deteriorating the thermal insulation ability of YSZ coating. It also increases the elastic modulus due to microstructure densification, causing the decrease of the strain compliance of YSZ coating [7][8][9].…”