The Effect of Negative Bias Voltage on the Microstructure and Hot Corrosion Behavior of Heat-treated NiCoCrAlY Coatings Applied Via the Cathodic Arc Evaporation Physical Vapor Deposition Method

NiCoCrAlY coatings have good corrosion and oxidation resistance in gas turbine blades at elevated temperatures. This study used the cathodic arc evaporation physical vapor deposition technique to apply the coatings on a Mar-M-200 nickel superalloy. The impact of a negative substrate bias voltage (0-300 V) and subsequent vacuum heat treatment on the microstructure, morphology, and phase transformation were investigated using EDX, EPMA, XRD, and SEM equipment. To evaluate the hot corrosion behavior of specimens, average mass change of substrate and coatings were considered in a mixture of Na2SO4-25 wt.% K2SO4 salt at 950 degrees C. Based on experimental results, the main phases at lower biases of 0 V and - 120 V were gamma '-NI3Al, gamma-Ni, and alpha-Co. In contrast, higher negative bias voltages revealed sharper and more substantial diffraction peaks of the alpha-Cr and beta-(Co, Cr)Al phases were revealed at. In floating conditions (0 V bias), the average crystallite size of samples was approximately 26 nm. The crystallite sizes were increased by enhancing the bias voltage up to - 300 V. The heat treatment of coatings achieves lower porosity and a smoother surface due to diffusion and annealing effects. Furthermore, heat-treated samples demonstrated larger crystallites. The EDS analysis revealed that the heat-treated specimens contained more Al and Y elements and lower Ni, Co, and Cr elements than the as-deposited specimens. After 30 h, the heat-treated coated sample at - 300 V bias exhibited only a short weight increment of 3.69 mg/cm(2) and showed the highest corrosion resistance against hot corrosion of all specimens.