Response surface methodology (RSM) optimization of titanium aluminide (TiAl-γ) coatings prepared by the electro-spark deposition (ESD) method

This study investigated the relationship between the process parameters on the thickness of titanium aluminide (TiAl-γ) coatings deposited using the electro-spark deposition (ESD) method on Ti6Al4V substrates. Using the design of experiments (DOE) approach and the response surface methodology (RSM), a set of experiments were designed to evaluate the effects of peak voltage, spark frequency, and electrode rotation speed on the thickness of ESD coatings. Surface morphology and cross-sectional view of coatings were analyzed using scanning electron microscopy (SEM) equipped with energy-dispersive spectroscopy (EDS). To verify the phases formed during the deposition process, X-ray diffraction (XRD) was utilized. The microhardness of the surface and the cross-sectional profile of coatings were evaluated using a Vickers tester. Electrochemical tests were applied to analyze the corrosion behavior of the sample with the maximum coating thickness, the substrate, and the electrode in a 3.5 wt% NaCl solution. A second-order polynomial equation with interaction terms was developed to describe the relationship between deposition parameters and coating thickness, demonstrating high predictive accuracy (R2 = 0.984). The maximum coating thickness of 18 μm was achieved with higher peak voltage and lower spark frequency. The average surface microhardness of the coated samples (∼600 HV) was 1.5 times higher than that of the electrode (317 HV) and the substrate (360 HV). The maximum microhardness observed within the cross-section reached ∼1400 HV, decreasing gradually with depth. Surface cracks, inherent to ESD coatings, exhibited a calculated surface density of approximately 0.017 μm−1. Corrosion tests showed that the ESD coating remarkably improved the resistance of Ti6Al4V to local corrosion in chloride-rich environments.