Characterization of boron and aluminum coatings grown on surface of cobalt-based haynes 25 superalloy

Cobalt (Co) based superalloys maintain their mechanical properties up to high temperatures; however, their low surface hardness leads to unsatisfactory wear resistance in abrasive environments. Thermo-chemical surface treatments are utilized to improve this aspect. Through these processes, the surface hardness of cobalt-based superalloys can be increased, enhancing their wear, corrosion, and oxidation characteristics. In this thesis, three different thermal chemical coatings (boronizing, aluminizing, and boron-aluminizing) applied on Co-based Haynes 25 superalloy are examined for their characteristic properties and their effects on high-temperature wear behavior. Coating processes were conducted at 950°C for 3 hours in a controlled atmosphere. Characterization studies were carried out using scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), X-ray diffraction (XRD) analysis, microhardness, and high-temperature wear tests. The thickness values of the obtained boride, aluminide, and boride-aluminide coating layers were determined as 140±1.50 µm, 37.58±2.85 µm and14.73±1.71 µm, respectively, with hardness values of 12.23±0.9 GPa, 26.34±2.33 GPa, and 23.46±1.29 GPa. The hardness of the coating layers resulted in reduced wear volume losses at both room temperature and 500°C. The best wear resistance at room temperature was achieved in the boronized sample due to its high hardness, while at 500°C, the best wear resistance was obtained in the boron-aluminized sample due to the reduction of aluminum oxidation within the boron-aluminide layer and the lubricating effect of boron. This indicates that the presence of some aluminum in the boride layers enhances their high-temperature wear resistance. While the coated samples experienced abrasive wear at room temperature, the wear mechanism at 500°C shifted to oxidative-assisted adhesive wear.