Restrained TGO growth in YSZ/NiCrAlY thermal barrier coatings by modified laser remelting

Abstract

Thermal barrier coatings (TBCs) are widely applied in high temperature gas turbines and jet engines to protect the superalloy components. Due to oxygen transfer through topcoat and the oxidation of bond-coat alloy, a thin layer of thermally grown oxide (TGO) will generate between the ceramic topcoat and the metallic bondcoat of TBCs after long-term operation. Cracking and spalling are often observed at the topcoat-TGO and TGO-bondcoat interfaces since the thickening of TGO layer may lead to strong tensile stress. Previously, laser remelting process has been developed as an effective way in restraining the TGO growth. In the present research, preheating laser remelting and sol-gel repairing laser remelting were respectively employed to further decelerate the TGO growth by reducing the number density and average size of vertical cracks in the topcoat. After 100h isothermal oxidation, the TGO growth rate of the modified laser remelted specimens are much lower than the conventional laser remelted specimens, which shows the potentiality of preheating and sol-gel repairing as effective promotions of laser remelting process in restraining the TGO growth.

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

“…Meanwhile, microcracks generate from the stress concentration of the high cooling rate, holes and voids are produced from an insufficient overlap and residual gas, and a rough surface and the looser microstructure are presented. The AS TBCs surface roughness is 10.8 µm (Sa), as shown in Figure 1b, and it may increase the oxygen contact area and accelerate the interface oxidation reaction between the ceramic coating and bonding coating [34]. As shown in Figure 1c, the LR TBC surface is smooth and flat, except for the network cracks generated from the laser scanning melting and Ar rapid cooling [35], and the roughness decreases to 0.4 µm (Sa), as shown in Figure 1d.…”

Thermal Protection System and Thermal Management for Combined-Cycle Engine: Review and Prospects

“…For the thermal barrier coating system, in the extreme high temperature environment caused by hypersonic speed, the porosity of YSZ coating will decrease, which will then lead to coating failure. Thermally grown oxide (TGO) produced between the surface layer and adhesive layer of the thermal barrier coating is not conducive to the stability of the thermal barrier coating, and due to temperature changes, misfit strain and other issues caused by differences between thermal expansion coefficients of various coatings are obvious in combined propulsion systems [30]. In view of the shortcomings of current technologies applied in combined propulsion for hypersonic aircrafts, related researches have been carried out recently on the thermal protection system, and the latest research progress was summarized in Sections 2.2-2.4.…”

A comparative study of calcium–magnesium–aluminum–silicon oxide mitigation in selected self-healing thermal barrier coating ceramics

“…A major challenge for TBCs is that their porosity acts as a pathway for CMAS (calcium-magnesium-alumina-silicon oxide) infiltration and oxygen diffusion [7]. At elevated temperatures, CMAS becomes molten [8] and reacts with the TBC surface [7]. The molten CMAS also penetrates into the porous YSZ structure and reacts within while a fraction adheres to the topcoat [7].…”

“…At elevated temperatures, CMAS becomes molten [8] and reacts with the TBC surface [7]. The molten CMAS also penetrates into the porous YSZ structure and reacts within while a fraction adheres to the topcoat [7]. These liquid-solid interactions lead to the degradation of the TBC structure resulting in higher thermal conductivity and coating spallation [9,10].…”