Ceramic Top Coats of Plasma-Sprayed Thermal Barrier Coatings: Materials, Processes, and Properties

Abstract

The ceramic top coat has a major influence on the performance of the thermal barrier coating systems (TBCs). Yttria-partially-stabilized zirconia (YSZ) is the top coat material frequently used, and the major deposition processes of the YSZ top coat are atmospheric plasma spraying and electron beam physical vapor deposition. Recently, also new thermal spray processes such as suspension plasma spraying or plasma spray-physical vapor deposition have been intensively investigated for TBC top coat deposition. These new processes and particularly the different coating microstructures that can be deposited with them will be reviewed in this article. Furthermore, the properties and the intrinsic-extrinsic degradation mechanisms of the YSZ will be discussed. Following the TBC deposition processes and standard YSZ material, alternative ceramic materials such as perovskites and hexaaluminates will be summarized, while properties of pyrochlores with regard to their crystal structure will be discussed more in detail. The merits of the pyrochlores such as good CMAS resistance as well as their weaknesses, e.g., low fracture toughness, processability issues, will be outlined.

Thermal cycling performances of multilayered yttria‐stabilized zirconia/gadolinium zirconate thermal barrier coatings

“…In the SPS spraying process, molten GZO droplets impacted on relatively cold substrates (200°C-500°C), so that they were cooled down very quickly and solidified as a metastable defect fluorite phase. 6,11 During the thermal cycling test, the surface temperature of the tested samples reached 1400°C. At such high temperature, the metastable fluorite phase underwent a phase transformation into the equilibrium pyrochlore phase.…”

“…The state of art material used for top coats is 6‐8 wt% yttria‐stabilized zirconia (YSZ). It has the exceptional combination of desired properties, such as a high melting point (2700°C), a relatively low thermal conductivity (2.6 W/mK), a high coefficient of thermal expansion (CTE, 11 × 10 −6 K −1 ) and a high fracture toughness (~3 MPa m 1/2 ) . However, there is an upper limit of the long‐term application temperature which is about 1200°C.…”

“…Besides the phase transformation, accelerated sintering of YSZ also gives rise to early spallation of the TBCs . For these reasons, a number of ceramic materials have been studied to search for a better TBC material than YSZ …”

“…It has the exceptional combination of desired properties, such as a high melting point (2700°C), 2 a relatively low thermal conductivity (2.6 W/mK), 3 a high coefficient of thermal expansion (CTE, 11 × 10 −6 K −1 ) 4 and a high fracture toughness (~3 MPa m 1/2 ). [5][6][7] However, there is an upper limit of the longterm application temperature which is about 1200°C. Above this temperature, YSZ undergoes a phase transformation accompanied by a considerable volume change (~4 vol%) 8 which is destructive for TBCs.…”

“…9 For these reasons, a number of ceramic materials have been studied to search for a better TBC material than YSZ. 2,6,10 Gadolinium zirconate (Gd 2 Zr 2 O 7 , GZO) as a very promising top coat material has attracted a lot of interest for the last two decades. It has a high melting point (2570°C), 6,11 very good phase stability, 12 a considerably low thermal conductivity (1.2 W/mK), 13,14 excellent calcium-magnesium-alumino-silicates (CMAS) infiltration resistance [15][16][17] and a high CTE (10.5 × 10 −6 /K) 18 which is close to that of YSZ.…”