Phase stability, thermo-physical property and thermal cycling durability of Yb2O3 doped Gd2Zr2O7 novel thermal barrier coatings

Oxidation is one of the failure mechanisms of gas turbines under operating temperature. Yttria-stabilized zirconia (YSZ) is well-known ceramic topcoat material of thermal barrier coatings (TBCs), but it has some drawbacks. From this point of view, other new materials such as rare earth zirconate are interesting. In the present research, single-layer Gd2Zr2O7 (GZO), and GZO/YbSZ (Gd2Zr2O7 + 50 wt% YbSZ), and double-layer YSZ + GZO/YbSZ (YSZ/Gd2Zr2O7 + 50 wt% YbSZ) topcoats as TBC were deposited by air plasma spraying. To estimate the phase stability of homogeneous mixtures of GZO (pyrochlore) and YbSZ (tetragonal), heat treatment was conducted at 1400 °C and showed complete conversion to a single-cubic (fluorite) phase after 100 h. The as-sprayed TBCs were subjected to oxidation for different times at 900 °C, and their features were investigated by different analysis techniques (SEM and XRD). The microstructural analysis indicated that the thickness of the thermally grown oxide layer was less for GZO coating due to reduced oxygen diffusion. Conversely, because of the lower fracture toughness and lower thermal expansion of GZO, it had a considerably less lifetime. It was concluded that coatings with the new composition GZO/YbSZ exhibited higher oxidation resistance and longer lifetime than GZO coatings. 

Oxidation Behavior of Single-Layer Gd_2Zr_2O_7, and Gd_2Zr_2O_7/YbSZ, and Double-Layer YSZ + Gd_2Zr_2O_7/YbSZ Thermal Barrier Coatings

Herein, NiCr was coated on γ-TiAl via the hollow cathode glow discharge, whereas yttria stabilized zirconia (YSZ) was coated on the NiCr by multi-arc ion plating. A strong bonding between the two coatings and substrate, i.e., NiCr, YSZ, and γ-TiAl occurred. However, the surface of YSZ coating displayed macroparticles of different sizes, which affected the oxidation resistance. High-temperature oxidation behavior of YSZ/NiCr coating was investigated at 850, 950, and 1050 °C. The results showed that the oxidation kinetics of YSZ/NiCr coating followed the parabolic law. Moreover, the YSZ coating did not showed a phase transition, consisting of t-ZrO2 and c-ZrO2. After 100 h of oxidation at 850 and 950 °C, macrocracks appeared on the surface of macroparticles. Yet, a thermally grown TGO oxide layer of Cr2O3 with a thickness of 2-4 μm effectively inhibited the oxidation of YSZ/NiCr coating. However, when the temperature increased to 1050 °C, the number and size of microcracks on the macroparticles surface increased and reached the YSZ coating surface from different directions, resulting in a high degree of oxidation. The oxidation kinetics and oxide scales formed during the test indicated that the YSZ/NiCr coating effectively improved the oxidation resistance of γ-TiAl at 850, 950, and 1050 °C. However, since the microcracks formed on the macroparticles caused a severe oxidation, the effect of the macroparticles on the oxidation of YSZ/NiCr coating at high temperature could not be ignored.

Oxidation Mechanism of YSZ/NiCr Coating Prepared by Hollow Cathode Glow Discharge Phenomenon and Multi-arc Ion Plating

High energy mechanical-milling induced modifications in structural and microstructural properties of Gd2Zr2O7

Gd2Zr2O7 ceramic is considered as a potential candidate for thermal barrier coatings. Here we present hot corrosion behavior of single layer Gd2Zr2O7 (GZO), Gd2Zr2O7 + 50 wt% YbSZ (GZO/YbSZ), YSZ and double-layer YSZ/Gd2Zr2O7 + 50 wt% YbSZ (YSZ + GZO/YbSZ) coatings prepared by air plasma spraying on Inconel 625 substrates. Prior to hot corrosion the fracture toughness of the coatings was assessed by Vickers indentation showing a variation between 0.6 and 2.3 MPa.m1/2 where the GZO/YbSZ coating exhibited the highest fracture toughness. The hot corrosion test was conducted by exposing the coatings to an equimolar mixture of Na2SO4 and V2O5 at 750 °C. After the hot corrosion test, the type of phases, chemical composition, microstructure and crack formation were investigated. Formation of GdVO4 and YVO4 was observed as the corrosion products in the GZO containing coatings and the YSZ coating, respectively. All coatings showed the formation of monoclinic zirconia after the corrosion test. The Gd2Zr2O7 + 50 wt% YbSZ (GZO/YbSZ) coating showed the best hot corrosion resistance due to reduced reactivity and enhanced fracture toughness and represents a new composition with properties promising for TBC applications.

Hot corrosion of Gd2Zr2O7, Gd2Zr2O7/YbSZ, YSZ + Gd2Zr2O7/YbSZ, and YSZ thermal barrier coatings exposed to Na2SO4 + V2O5

Doped Gd2Zr2O7 materials have interesting properties as thermal barrier coatings (TBC) to replace the YSZ topcoats traditionally used. Here we investigate the thermomechanical properties and phase relations of Gd2Zr2O7 (GZO) alloyed with 5 mol% Yb2O3 stabilized ZrO2 (YbSZ) in the composition range (Gd2Zr2O7)1−x(YbSZ)x, 0 ≤ x ≤ 0.98. With increasing YbSZ content, phase transformations from ordered to disordered pyrochlore to fluorite and tetragonal structures were observed. The thermal expansion coefficient (TEC) and Vickers hardness were correlated showing a maximum hardness (~ 11.5 GPa) and minimum TEC at x = 0.82. At 1000 °C, the TEC for the end members, x = 0 and 0.98, were 11.4 and 11.3 × 10–6 K−1, respectively. The fracture toughness, KIC, showed an average value around 1.5 MPa m0.5 for x ≤ 0.93 and increased significantly at x = 0.98 reaching 5.4 MPa m0.5 due to the presence of a ferroelastic phase. For TBC applications, compounds with x = 0.98 show promise due to high TEC and high KIC. Fig. 5a summarize the most important results in the manuscript. Showing a significant increase in fracture toughness for compositions with x=0.98.

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Phase relations and thermomechanical properties of (Gd2Zr2O7)1−x(YbSZ)x based thermal barrier coatings (0 ≤ x ≤ 0.98)

Piezoelectric materials are widely used in electromechanical coupling components including actuators, kinetic sensors, and transducers, as well as in kinetic energy harvesters that convert mechanical energy into electricity and thus can power wireless sensing networks and the Internet of Things (IoT). Because the number of deployed energy harvesting powered systems is projected to explode, the supply of piezoelectric energy harvesters is also expected to be boosted. However, despite being able to produce green electricity from the ambient environment, high-performance piezoelectrics (i.e., piezoelectric ceramics) are energy intensive in research and manufacturing. For instance, the design of new piezoceramics relies on experimental trials, which need high process temperatures and thus cause high consumption and waste of energy. Also, the dominant element in high-performance piezoceramics is hazardous Pb, but substituting Pb with other nonhazardous elements may lead to a compromise of performance, extending the energy payback time and imposing a question of trade-offs between energy and environmental benefits. Meanwhile, piezoceramics are not well recycled, raising even more issues in terms of energy saving and environmental protection. This paper discusses these issues and then proposes solutions and provides perspectives to the future development of different aspects of piezoceramic research and industry. 

https://onlinelibrary.wiley.com/doi/pdfdirect/10.1002/gch2.202300061

Mohadese Tabeshfar

Papers

Hot corrosion of Gd2Zr2O7, Gd2Zr2O7/YbSZ, YSZ + Gd2Zr2O7/YbSZ, and YSZ thermal barrier coatings exposed to Na2SO4 + V2O5

Phase relations and thermomechanical properties of (Gd2Zr2O7)1−x(YbSZ)x based thermal barrier coatings (0 ≤ x ≤ 0.98)

Oxidation Behavior of Single-Layer Gd_2Zr_2O_7, and Gd_2Zr_2O_7/YbSZ, and Double-Layer YSZ + Gd_2Zr_2O_7/YbSZ Thermal Barrier Coatings

Toward Ecofriendly Piezoelectric Ceramics—Reduction of Energy and Environmental Footprint from Conceptualization to Deployment