Mechanical characterization and thermal behavior of HVOF-sprayed bond coat in thermal barrier coatings (TBCs)

The mechanical properties, hardness H and modulus E, of thermal barrier coatings (TBCs) have been determined as a function of the thickness of bond coats (0.08, 0.14, and 0.28 mm) prepared using a high-velocity oxygen fuel (HVOF) thermal spray process. The top coatings were fabricated by an air plasma spray (APS) process. Behavior in a given thermal environment was assessed from the H and E values, and from estimates of the contact damage. Thermal fatigue tests were conducted at T = 950 and 1100 °C using dwell times of 10 and 100 h at T = 950 °C, and 10 hr for T = 1100 °C. The formation of the thermally grown oxide (TGO) layer is influenced by both the temperature and the dwell time, but is independent of the thickness of the bond coat. The H and E values of the top coatings and the bond coats, respectively, show an abrupt increase after thermal fatigue, with a discontinuity occurring at the interface between the bond coat and the top coating, while the thickness of the bond coat plays a lesser role in influencing the mechanical properties. The evolution of contact damage at the subsurface and cyclic fatigue of the top coating has been investigated using Hertzian indentation. After the thermal fatigue tests, the contact damage is enhanced, while radial cracking on the top surface is suppressed. The contact damage is mainly affected by the temperature, showing severe damage in the substrate with decreasing thickness of the bond coat. The effects of the thermal fatigue condition on mechanical properties, contact damage, and cyclic fatigue are discussed and related to the resintering of the top coating and the formation of the TGO layer.