Fracture behavior and thermal durability of lanthanum zirconate-based thermal barrier coatings with buffer layer in thermally graded mechanical fatigue environments

Abstract

The effects of buffer layer on the fracture behavior and lifetime performance of lanthanum zirconate (La 2 Zr 2 O 7 ; LZO)-based thermal barrier coatings (TBCs) were investigated through thermally graded mechanical fatigue (TGMF) tests, which are designed to simulate the operating conditions of rotating parts in gas turbines. To improve the thermal durability of LZO-based TBCs, composite coats consisting of two feedstock powders of LZO and 8 wt.% yttria-doped stabilized zirconia (8YSZ) were prepared by mixing different volume ratios (50:50 and 25:75, respectively). The composite coat of 50:50 volume ratio was employed as the topcoat, and two types of buffer layers were introduced (25:75 volume ratio in LZO and 8YSZ, and 8YSZ only). These TBC systems were compared with a reference TBC system of 8YSZ. The TGMF tests with a tensile load of 60 MPa were performed for 1000 cycles, at a surface temperature of 1100 C and a dwell time of 10 min, and then the samples were cooled at room temperature for 10 min in each cycle. For the single-layer TBCs, the composite topcoat showed similar results as for the reference TBC system. The triple-layer coating (TLC) showed the best thermal cycle performance among all samples, suggesting that the buffer layer was efficient in improving lifetime performance. Failure modes were different for the TBC systems. Delamination and/or cracks were created at the interface between the bond and topcoats or above the interface in the single-layer TBCs, but the TBCs with the buffer layer were delaminated and/or cracked at the interface between the buffer layer and the topcoat, independent of buffer layer species. This study allows further understanding of the LZO-based TBC failure mechanisms in operating conditions, especially in combined thermal and mechanical environments, in order to design reliable TBC systems.

Effect of thermal cycling frequency on the durability of Yb-Gd-Y-based thermal barrier coatings

“…In another word, the appearance of narrow cracks is beneficial to prevent the vertical cracks propagating during the process of stress relief. In our previous study [28], the blended LZO-YSZ based TBCs with RP-YSZ buffer layer generated more vertical cracks than YGYZ-based TBCs. Therefore, the YGYZ-based TBCs showed a better thermal durability in the low-frequency cycling TGMF test.…”

Mechanical properties of lanthanum zirconate-based composite thermal barrier coatings

“…However, due to its low coefficient of thermal expansion, high thermal stresses occur near the interface between the top and bond coats, causing delamination and low durability in cyclic thermal environments. Vassen [4,[9][10][11] and our group [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26] have developed layered LZ based coatings to mitigate the issue. Double-ceramic-layer (DCL) TBCs have been proposed, which include a top ceramic layer and a thin ceramic buffer layer.…”

Mechanical Properties of Layered La2Zr2O7 Thermal Barrier Coatings

“…Sivakumar et al reported that the adhesive strengths were 6.63 MPa, 6.17 MPa, and 3.73 MPa, which corresponded to the plasma sprayed La2Zr2O7 coatings with the porosity of 16%, 21% and 28%, respectively [11]. Lyu et al investigated the cycling performance of La2Zr2O7/8YSZ composite coating in TGMF tests, and indicated that La2Zr2O7/8YSZ buffer layer improved the TGMF cycling performance [12]. A few recent reviews of La2Zr2O7 based coating materials were given by Bakan [13] and Zhang [14].…”