Inspired by the high temperature capability and excellent thermal and mechanical properties of LZA( LaZnAl11O19), we have prepared a LZA coating on a DZ125 superalloy substrate and the cyclic durability was evaluated at 1100℃. Fig.1 shows the micro-structure evolution of LZA TBC before and after thermal cycling. As shown in Fig.1a, the thickness of the LZA coating is 250 um and the cross-section microstructure of the as-sprayed LZA coating indicates the good melting condition of the ceramic powders during plasma spraying. The selected region with a higher magnification shows that some pores and cracks also exist, which is typical of APS coatings. It should be noted that some horizontal cracks can be observed in the LZA coating, which may be induced by the tensile residual stress generated by the rapid cooling of the molten droplets when spreading out onto the cold substrate during plasma spraying. The horizontal cracks may have some adverse effect on the cyclic durability of the LZA TBC. Fig.9b shows the surface photo and cross-sectional SEM micrograph of the thermally cycled LZA coating. After 50 cycles,a large area of the coating peels off, which accounts for more than 20% of the sample area. The failure of the LZA coating was caused by the crack propagation in the LZA coating next to and along the LMA/bond coat interface, as shown in Fig.9b.
The XRD pattern of the LZA coating after thermal cycling is shown in Fig.2. No secondary phase is generated except for a better crystalline state of the LZA coating. From the fractured cross-section SEM micrograph, it can be observed that lots of platelet-like grains were recrystallized from the molten lamel-lae of the LZA coating during thermal cycling, which is consist-ent with the XRD result. Generally, the formation and growth of the thermally grown oxides(TGO) are deemed to be the most important phenomenon responsible for TBC failure.However, the formation of TGO is not obvious in the present study, which maybe resulted from the relatively short thermal cycling lifetimes and the low oxygen diffusion through the magnetoplumbite oxides.51 Therefore, the TGO is absolutely not a major factor responsible for the spallation of LZA TBCSin this work and the premature failure can be mainly attributed to the low thermal expansion coefficient and recrystallization of the LZA coating. As mentioned above, the average TEC of the LZA coating is less than 6.0× 106 K1, which is much lower than that of the MCrAlY bond coat(13-17×10-6K-1).As a result, large interface thermal stresses generate during thermal cycling due to the thermal expansion mismatch between the ceramic top coat and metallic bond coat. In addition, the parallel contraction of the LZA coating occursduring the recrystallization process and then plane tensile stresses will develop in the LZA coating due to the restriction of the bond coat. On the other hand, the growth and rearrangement of the platelet-like grains will result in the formation of interface deficiencies, which would reduce the bond strength between the ceramic topcoat and the bond coat to some extent. When the thermal stresses are exceeding the threshold for crack, interface cracks begin to form, which finally cause LZA coating spallation.
Fig.1 Photos of the surface and SEM micrographs of the LaZnAl11O19 coating:(a) before and(b) after thermal cycling.
Fig.2 XRD patterns of the LaZnAl11O19 coating annealed at 1300 ℃ for different time periods and after thermal cycling.
本文由桑尧热喷涂网收集整理。本站文章未经允许不得转载；如欲转载请注明出处，北京桑尧科技开发有限公司网址：http://www.sunspraying.com/
|
