Bonding Strength of Top Coat and its Fracture Mechanism in Thermal Barrier Coatings with Thermal Fatigue

Abstract

The bonding strength of top coat in thermal barrier coatings (TBCs) with different bond layers has been measured with thermal fatigue and its fracture behavior has been investigated considering the effects of a thermally grown oxide (TGO) layer and resintering of the top coat. Two kinds of TBCs with different thicknesses of approximately 80 and 280 μm in bond layer were prepared by two different methods of air plasma spray (APS) and high velocity oxygen flow (HVOF). Top layer (3Y-TZP) was coated on both bond layers using the APS process. Thermal fatigue tests were conducted at temperature of 1100oC with dwell time of 10 hr, which tests were done till 20 cycles. The HVOF process provides a relatively homogeneous microstructure in the bond layer, compared with the APS process. The formation of TGO layer is influenced by the exposed time, independent of the bond layer thickness and the bond layer species. TBCs with the bond layer prepared by the APS process show higher bonding strength than those by the HVOF process in as-prepared TBCs (before thermal fatigue), showing 15 – 18 MPa for the APS process and 13 – 16 MPa for the HVOF process. Whereas the fracture in TBCs with the HVOF bond coat are originated at the interface between the top coat and the bond coat, the fracture within the top coat in TBCs with the APS bond coat However, after the thermal fatigue, the bonding strength value is increased to 18 – 20 MPa in both cases even though the TGO layer is formed, showing the fracture paths within the top coat in both cases.