PhD Thesis Colloquium: Mr. Sachin Kumar (05/12/23)
Thesis title:
Extraction of Bond Coat Mechanical Properties using High Temperature Digital Image Correlation
Faculty advisor(s):
Prof. Vikram Jayaram
When?
05th December, 2023 (Tuesday), 11:00 AM (India Standard Time)
Where
KI Vasu Auditorium, Department of Materials Engineering
Abstract:
Gas turbine engines used in an aircraft or in power generation widely use nickel-based superalloys in their hot sections such as combustion chambers, turbine blades, etc. These components in the hot section use thermal barrier coating (TBC) which reduce the temperature experienced by the superalloy. A typical TBC is made of two layers: an oxidation resistant bond coat such as diffusion aluminide or overlay MCrAlY coating, and a ceramic top coat, typically yittria stabilized zirconia (7YSZ). While oxidation and corrosion protection are the key function of bond coats, their mechanical behaviour play a key role in the spallation of the TBC, particularly in thermal cycling scenario like those in aircraft engines. The cyclic thermal loading leads to undulation at the bond coat surface known as rumpling. However, limited information on the thermos-mechanical properties is available in literature. Attempts to measure these properties have been inhibited by the limited thickness of these coatings. Studies to measure these properties using ingots cast with the same chemical composition are undermined by the fact that the microstructure of the bond coat is substantially different than that of the casting. In the present study, a small-scale testing method based on digital image correlation is performed. This testing method is first validated and then used to characterize the thermo-mechanical properties of NiCoCrAlY (MCrAlY type) of bond coats. Properties like CTE, Modulus, Poisson’s ratio, creep parameters are obtained. The technique used for deposition of bond coat can also play an important role in the mechanical properties. A comparative study of the mechanical behaviour of two popular process of deposition: 1) High Velocity Oxy-Fuel (HVOF), and 2) Vacuum Plasma Spray process is discussed. Finally, to understand the difference in behaviour of the two types of coating, microstructural studies were performed.