Effect of Humidity and Temperature on PVD TiAlN-Coating Wear.

Gas turbine blades and disks undergo wear at high temperatures at dovetail joints where tolerances are very small. Thin hard coatings are known to enhance the wear resistance of the superalloy components minimally influencing the tolerance levels. However, fundamental understanding of the coating's wear mechanisms operating in these harsh conditions is not well understood. In this study, wear tests are performed to understand the wear mechanisms that operate in the temperature range from RT up to 800 °C for thin hard TiAlN coating using simple wear geometry eliminating any influence of wear debris. It is challenging to measure wear of thin hard coatings especially at elevated temperatures but important nevertheless. A coated ball on disk geometry with rough alumina as counterface is used for wear studies to understand exclusively the influence of humidity and temperature coating wear behavior. Cathodic arc evaporation, a physical vapor deposition technique is used to deposit TiAlN coatings on heat-treated IN718 substrates and characterized with, XRD, EPMA, TEM, SEM, nanoindentation, and FIB. The wear at room temperature shows scatter which has been ascribed to seasonal fluctuations in relative humidity. Further, wear results are shown to correlate with Young's equation for capillary condensation. Wear below 50 pct RH is essentially dry and constant up to 600 °C above which wear increases marginally upto 800 °C. The coefficient of friction shows a maximum at 400 °C, below which friction reduces due to increased adsorption of water vapor, while above 400 °C, TiO₂ forms on the surface to reduce the friction. The wear rate at 3 N load in the range of 50–800 °C is ~ 1 × 10⁻⁶ mm³/m/N. For 5 N load, the wear rate is same as for 3 N load upto 600 °C but doubles above 700 °C. The average contact pressure through the test is ~ 550 and 650 MPa which is almost twice the design contact pressure. The wear debris gets richer in Ti with increase in temperatures. The Al-rich TiAlN coatings deposited by cathodic arc evaporation (CAE) technique show a low and constant wear behavior over a wide range of temperatures and are ideally suited for the protecting the dovetail joints in gas turbines. [ABSTRACT FROM AUTHOR]