A Comparative Study on Characterization and High-Temperature Wear Behaviors of Thermochemical Coatings Applied to Cobalt-Based Haynes 25 Superalloys

This study investigated the characteristic properties of aluminizing, boronizing, and boro-aluminizing coatings grown on Haynes 25 superalloys and their effects on the high-temperature wear behavior. The coating processes were conducted in a controlled atmosphere at 950 & DEG;C for 3 h. Characterization studies were performed using scanning electron microscopy, energy-dispersive spectroscopy, X-ray diffraction analysis, nanoindentation testing, and high-temperature wear tests. It was determined that the thickness values of aluminide, boride, and boride-aluminide coatings were 140 & PLUSMN; 1.50 & mu;m, 37.58 & PLUSMN; 2.85 & mu;m, and 14.73 & PLUSMN; 1.71 & mu;m, and their hardness values were 12.23 & PLUSMN; 0.9 GPa, 26.34 & PLUSMN; 2.33 GPa, and 23.46 & PLUSMN; 1.29 GPa, respectively. The hardness of the coatings resulted in reduced wear volume losses both at room temperature and at 500 & DEG;C. While the best wear resistance was obtained in the boronized sample at room temperature due to its high hardness, the best wear resistance at 500 & DEG;C was obtained in the boro-aluminized sample with the oxidation-reduction effect of Al content and the lubricating effect of B content in the boro-aluminide coating. This indicates that the presence of aluminum in boride layers improves the high-temperature wear resistance of boride coatings. The coated samples underwent abrasive wear at room temperature, whereas at 500 & DEG;C, the wear mechanism shifted to an oxidative-assisted adhesive wear mechanism.