Hot Corrosion Behaviors of Cr2alc, Cr3C2, and Al4C3 Thin Film Coatings on Ni-201

Hot corrosion, also known as deposit-induced accelerated oxidation, is an insidious challenge for hardware within high-temperature systems, such as gas turbines in combined cycle power plants and aircraft engines. This hardware, generally consisting of nickel based super-alloys, often requires protective surface coatings to satisfy the requirements of both superior mechanical properties and high-temperature corrosion resistance.

M_n+1AX_n (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, Cr₂AlC has attracted considerable interest because of its excellent high-temperature oxidation and hot corrosion resistance.  The main objective of this study is to determine the effectiveness of Cr₂AlC, Cr₃C₂ and Al₄C₃ thin film coatings for the hot corrosion protection of Ni-201. Different thin film coatings were deposited via magnetron sputtering physical vapor deposition (PVD) before being exposed to air and 2 ppm SO₂ at both 700°C and 900°C for up to 200 hours to simulate hot corrosion conditions.

Phase compositions of each coating 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 coated specimens exhibited much better oxidation behavior as compared to the bare material. A continuous oxide scale formed on the coating surface during the initial oxidation stages. The oxide scale and coating itself acted as diffusion barriers blocking the further ingress of oxygen and protected the substrate alloy from oxidation. The oxidation mechanisms of the bare alloy and the coated specimens were proposed and compared based on the experimental results. The coating degradation can be attributed to the Al and Cr consumption by surface oxidation and inter-diffusion between the coating and the substrate.  Results and interpretations will be presented and discussed in context of improving mechanistic understanding of hot corrosion behaviors of MAX phase materials.