Fracture characteristics of thermal barrier coatings after tensile and bending tests

Abstract

Uniaxial tension and four-point bending tests were conducted on two-layer (non-functionally graded material) plasma-sprayed rectangular specimens, boasting a new, functionally graded material (FGM) layer. The top coat layer was composed of stabilized yttria-zirconia (YSZ), and the bond coat layer was NiCrAlY. The FGM was composed of five layers: 100% YSZ; 80% YSZ+20% NiCrAlY; 60% YSZ+40% NiCrAlY; 40% YSZ+60% NiCrAlY; and 20% YSZ+80% NiCrAlY. Fracture in the thermal barrier coating (TBC) system was examined by scanning electron microscopy (SEM), which showed that vertical multiple cracking first occurred in the top coat, followed by propagation of interface cracking between the top and bond coats. Spallation of non-FGM coatings occurred in tensile/bending experiments, but not for the FGM-coated specimens, with only localized delamination observed along the interface between FGM layers and the substrate. For specimens prepared with a gun–substrate distance of approximately 120 mm and gun operating power of approximately 32.5 kW, the interface fracture toughness, evaluated by a shear lag model for the uniaxial tensile test, was 0.94 and 0.67 MPa m 1/2 for type-A and -B coatings, respectively. Here, types A and B correspond to an average roughness of the bond coat surface of 12.8 μm and 6.8 μm, respectively. The interface fracture toughness evaluated by the Suo–Hutchinson model was 1.05–1.27 and 1.0–1.17 MPa m 1/2 for type-A and -B coatings, respectively. The results are very close to data obtained by the blister test method. For specimens prepared with a gun–substrate distance of 125–130 mm and gun operating power of approximately 38.5 kW, the interface fracture toughness, evaluated by the Suo–Hutchinson model for four-point bending, was 4.26–7.21 MPa m 1/2.