Thermal barrier coatings (TBCs), primarily composed of yttria-stabilized zirconia (YSZ), are critical materials for protecting the hot section components of gas turbine engines due to their high fracture toughness, melting point, and thermal stability. However, increasing operational temperatures has some challenges, particularly from molten CMAS (CaO-MgO-Al₂O₃-SiO₂) deposits that can penetrate TBCs, reduce their strain tolerance, and accelerate failure. The performance of YSZ deteriorates upon interaction with CMAS, leading to phase transformation and coating detachment. Research efforts focus on modifying YSZ with TiO₂, Cr₂O₃, or Al₂O₃ based on sacrificial protection strategies, enhancing resistance to CMAS via promoting the crystallization of compounds such as gehlenite (CaAl₂Si₂O₈). This study was to investigate the effect of Ti on the reactivity of sacrificial protective Al₂O₃ coatings with CMAS. Al and Ti layers were deposited on YSZ by magnetron sputtering and heat treatment to form mixed oxide layers to improve the protective efficacy against CMAS attack.

In this study, graphite was utilized as a substrate for YSZ coatings. After degreasing and cleaning, graphite substrates were roughened with alumina sand (WA22). A YSZ powder (KF231) was sprayed onto the substrates in Ar-H₂ as a plasma gasby a model Multicoat APS system (Oerlikon Metco Co., Switzerland) to deposit a YSZ coating with the thickness of 1500 μm. The graphite substrate was removed by heat treatment at 750 ℃ for 3 h. Subsequently, Al and Ti films were deposited on the YSZ surface via magnetron sputtering, and followed via annealing at 1000 ℃ for 2 h to form TiO₂-Al₂O₃ modified YSZ (TA-YSZ) coatings. An additional control group of Al₂O₃-modified YSZ (A-YSZ) without Ti deposition was also prepared.

CMAS powders with a composition of 33%CaO-9%MgO-13%AlO_1.5-45%SiO₂ (in mole fraction) were synthesized via melt quenching. The melting and crystallization temperatures of CMAS and its mixtures with Al₂O₃ and TiO₂ were characterized by differential scanning calorimetry (DSC). The thermal corrosion test was performed by painting the coated surface with a CMAS slurry and exposing it to 1300 ℃ for 5 min or 1250 ℃ for 2 h and 10 h (after 2 h heat treatment, the temperature was lowered in a furnace and then raised to 1250 ℃ for 2 h, and this was repeated for 5 times). The phase structure was determined by X-ray diffraction (XRD, Rigaku Smart Lab., Japan). The surface and cross-sectional microstructures of the corroded samples were analyzed by a model Phenom ProX field emission scanning electron microscope (SEM, Thermo Scientific, Netherlands) equipped with an energy-dispersive X-ray spectroscopy (EDS) system. The corroded samples were embedded in epoxy resin, cut and diamond-polished before performing the SEM analysis of the coating cross-section. The phase structure of YSZ was determined by a model InVia-Reflex Fourier-transform Raman spectroscopy (Renishaw Co., UK).

Al and Ti films are sequentially deposited on the surface of YSZ coatings via magnetron sputtering. Al film exhibits a typical physical deposition structure. After heat treatment, the Ti element shows a dotted band above Al layer and forms an oxide layer. The results of DSC analysis reveal that the crystallization and melting characteristic temperatures of the three powder samples (i.e., CMAS, CMAS+Al₂O₃, and CMAS+Al₂O₃+TiO₂) shift towards lower temperatures in such an order. This indicates that the addition of Al₂O₃ and TiO₂ effectively reduces the crystallization temperature of CMAS and affects its melting characteristics.

TA-YSZ and A-YSZ coatings coated with CMAS and subjected to treatments at 1300 ℃ for 5 min or 1250 ℃ for up to 2 h and 10 h show that Ti promotes reactions between CMAS and Al₂O₃ to form high-melting-point crystalline phases such as gehlenite (CaAl₂Si₂O₈) and spinel (MgAl₂O₄). The introduction of Ti significantly enhances the interfacial reaction kinetics between the coating and the CMAS melt. Ti facilitates the formation of high-melting-point crystalline phases. Also Ti accelerates the crystallization process of corrosion products, forming a dense reaction barrier layer that effectively consumes molten CMAS and inhibits its penetration into the YSZ substrate. Moreover, the time and spatial distribution characteristics of the reaction products can occur during corrosion testing due to the different reaction kinetics characteristics of high-melting-point crystalline phases. Under the sacrificial protection mechanism of the surface coating, no phase transformation appears in the YSZ substrate.

YSZ coatings prepared by the atmospheric plasma spraying method were subjected to magnetron sputtering to deposit Al and TiAl films on their surface, which were then in-situ oxidized to form Al₂O₃ and TiO₂-Al₂O₃ (TA-YSZ) layers. The synergistic effect of Ti and Al on the enhancement of the CMAS corrosion resistance of YSZ coatings was investigated. The results showed that Ti significantly accelerated the reaction between CMAS and Al₂O₃, leading to the formation of high-melting-point products such as gehlenite and spinel, and promoted the crystallization of corrosion products. These actions effectively hindered the penetration of molten CMAS into the YSZ coating interior.