Hot Corrosion Behavior of Multilayered Titanium Aluminum Carbide/Yttria-Stabilized Zirconia Coatings for Silicon Carbide

Abstract

Economic and environmental concerns are the main driving force behind the ever increasing demand for higher gas turbine engine inlet temperatures critical for meeting higher efficiency goals. Turbine materials, while in operation, are exposed to corrosive deposits of molten Na2SO4 produced from the high temperature reaction of environmental NaCl salt with sulphur resulting from fuel combustion. This phenomenon is known as “hot corrosion” and may lead to catastrophic failure of structural components. New light-weight and high-strength, high-temperature materials are needed to replace performance-limited metallic-based turbine components. Ceramic matrix composite (CMC) materials are attractive candidates for turbine hot-section components; however, direct exposure to combustion environments leads to degradation via corrosion, volatilization and associated surface recession. To protect against these phenomena, multilayered thermal and environmental barrier coatings are required to increase durability through cyclic operation to temperatures in excess of 1000°C. Mn+1AXn (abbreviated as MAX, where M is an early transition metal, A is an IIIA- or IVA-group element, and X is C or N) phases have received increasing attention because of their unique combination of properties including excellent thermal shock resistance and damage tolerance. Among them, Ti2AlC has attracted considerable interest because of its excellent high-temperature oxidation and hot corrosion resistance. Yttria Stabilized Zirconia (YSZ) is instead recognized as an efficient thermal barrier coating (TBC). The main objective of this study is to determine the effectiveness of a combined thin film coating (~5 μm) comprised of a MAX phase sublayer and a TBC top layer, specifically Ti2AlC coupled with YSZ. The thin film coatings were deposited via electron beam evaporation, physical vapor deposition (EB-PVD) on bulk SiC substrates used as prototypical material. Uncoated and coated samples were exposed to a simulated combustion environment gas mixture of air with 100 ppm SO2 after the deposition of 1 mg/cm2 of prototypical salt (Na2SO4) at 700 °C for up to 100 hours. The corrosion behavior of uncoated, Ti2AlC coated and Ti2AlC/YSZ coated samples were compared. Thermogravimetric analyses were performed to evaluate the development of the surface oxide layer in order to quantify the entity of the corrosion process. Results were then compared with those collected for samples exposed to simple oxidation conditions (air only). Phase compositions of coated and uncoated samples before and after exposure were assessed using X-ray diffraction (XRD) and their corresponding morphologies (surfaces and cross sections) analyzed with a scanning electron microscope (SEM) equipped with an energy dispersive x-ray spectroscopy (EDS) system. The corrosion mechanisms of the uncoated and coated specimens were proposed and compared based on the experimental results. Results and interpretations will be presented and discussed in the context of improving mechanistic understanding of hot corrosion behaviors of protective thin film coatings.