Design, Preparation, and Characterization of Graded YSZ/La₂Zr₂O₇ Thermal Barrier Coatings

Abstract

Large‐area spallation and crack formation during service are big problems of plasma‐sprayed thermal barrier coatings (TBCs), owing to their weak bond strengths and high residual stresses. Functional gradient TBCs with a gradual compositional variation along the thickness direction are proposed to mitigate these problems. In this paper, a six‐layer structured TBC composed of Y2O3 partially stabilized ZrO2 (YSZ) and La2Zr2O7 (LZ), was prepared by plasma spraying with dual powder feeding ports. This coating had a gradient composition and function. Thermal conductivity of the coating was comparable with that of a single LZ coating while the coefficient of thermal expansion was nearly equal to that of YSZ single coating. The experiment was conducted to compare the thermal shock resistance of a graded coating with a conventional YSZ/LZ double‐layer system. Changes in weight and morphology of specimens before and after thermal shock tests were analyzed. Results demonstrated that the thermal shock resistance of the graded coating was superior to the double‐layer coating. Typically, a barely visible pimple‐like spallation was present on the surface of the graded coating after 21 cycles. On the other hand, obvious delamination was observed for a double‐layer coating after six to seven cycles. Special focus was also placed on a comparative investigation of stresses that are closely related to spallation via the use of numerical simulation.

Thermophysical properties and cyclic lifetime of plasma sprayed SrAl12O19 for thermal barrier coating applications

“…It indicates that the failure of SrAl 12 O 19 /YSZ DCL coating always appears in YSZ layer close to the bond coat, whereas the interface between SrAl 12 O 19 layer and YSZ layer is still intact, indicating that the stress concentration caused by thermal expansion mismatch between the SrAl 12 O 19 coating and bond coat is eased by the presence of the YSZ buffer layer. Most of the DCL coatings reported in literature such as La 2 Zr 2 O 7 /YSZ, 25 Gd 2 Zr 2 O 7 /YSZ, 26 or SrZrO 3 /YSZ 44 coatings showed a failure at the interface between the two ceramic layers, which has never been the case in the present study. A possible explanation for the difference in failure mode between SrAl 12 O 19 /YSZ coating and the La 2 Zr 2 O 7 /YSZ coating could be the differences in their microstructure evolution and fracture toughness.…”

“…Since SrAl 12 O 19 coating showed no obvious phase transformation except the un‐reversible amorphous crystallization (Figure 2), thermal cycling failure of the SrAl 12 O 19 coating is interpreted to be mainly driven by the thermal expansion mismatch stress which can be expressed by the following equation 52 :where σ 0 is the thermal stress, Δ α is the CTE difference between the coating and the substrate, Δ T is the temperature difference, E and υ are the elastic modulus and Poisson ratio of the coating, respectively. The large CTE difference between SrAl 12 O 19 coating (~7.52 × 10 −6 K −1 ) and MCrAlY bond coat (13‐17 × 10 −6 K −1 ) 25 could cause a large interface thermal stresses according to Equation (). On the contrary, the formation of interface deficiencies due to the amorphous recrystallization would reduce the bond strength between SrAl 12 O 19 coating and MCrAlY bond coat.…”

“…19 To meet the requirement of further ultraefficient propulsion engine systems, the search for new materials that can withstand higher gas-inlet temperature has been intensified in the last few decades. 20 Refractory oxides such as rare-earth zirconates (R 2 Zr 2 O 7 , R = La, Nd, Sm, Gd), [21][22][23][24][25][26] rare-earth hafnates (La 2 Hf 2 O 7 , 27,28 Yb 3 Hf 4 O 12 29,30 ), Y 3 Zr 4 O 12 , 31 and LaMgAl 11 O 19 32 have been proposed as new T/EBC candidates. In particular, MgO-doped rare-earth hexaluminates RMgAl 11 O 19 (RMA) with a magnetoplumbite structure have attracted increasing attention due to their promising thermophysical properties and excellent thermal stability up to 1800°C.…”

Isothermal Oxidation TGO Growth Behaviors of Laser-Remolten LZO/YSZ Thermal Barrier Coatings

“…Figure 4b shows that both AS and LR TBC LZO surface phases after 100 h of isothermal oxidation are tetragonal ZrO 2 with a low-intensity diffraction peak, although the major diffraction peak still corresponds to the pyrochlore structure. The tetragonal ZrO 2 phase formation is attributed to the preferential volatilization of La 2 O 3 during the longtime oxidation at high temperature, and thus, the stoichiometric inhomogeneity forms in LZO [38]. This is not the reason for the LZO/YSZ TBC's failure during the high-temperature service, so the influence of the ceramic coating phase transformation on high-temperature oxidation resistance can be negligible [39].…”

Phase stability (at 1000°C) of hollow t‐ZrO2 fibers costabilized by lanthana and yttria

“…3 | RESULTS AND DISCUSSION3.1 | Phase analysis On extending the holding time to 50 hours, m-ZrO 2 begins to appear in the YSZ fibers in a small amount, but not in the LaYSZ fibers. Using a previously reported formula,16 the proportion of m-ZrO 2 was calculated to be 4.84%. When the holding time is extended to 100 hours, the characteristic peaks of m-…”