Evaluation of Shear Fatigue Delamination Strength of Porous Thermal Barrier Coatings by Torsion Pin-Test Method

Abstract

The shear fatigue delamination strength of some porous thermal barrier coatings (TBCs) has been evaluated using the torsion pin-test method. The porosities of the tested TBCs were 0-5, 5-10, 15-20 and 25-35%. The shear delamination strength of the TBCs was evaluated and the shear fatigue delamination strength (S–N) curves were obtained under cyclic torsional loading. The Young’s modulus of the TBCs was evaluated by using indentation tests and the corresponding numerical analyses, and expressed as a function of coating porosity. Finite element analyses, using the commercial nonlinear finite element analysis software MARC, showed that almost the same stress distributions could be observed around the interface edge between the top coat and the bond coat when delamination occurred, regardless of the pin diameter. Therefore, only the specimens of 4-mm pin diameter were used in the fatigue test. The shearing fatigue delamination strength was investigated under cyclic torsion without tension. If combined torsion and tension were applied, fracture would occur inside the top coating instead of adhesively at the bond coat interface. It was found that the normalized S–N curves, divided by the critical delamination stress due to simple torsion, coincided with each other. Next, the stress intensity factor (K) of the singular shear stress field around the interface edge was determined for various porous coatings under simple torsion, and the relationships between K and porosity Pv were derived. The strength (K) of the shear stress singularity field was adopted as the fatigue delamination criterion for the porous TBCs under cyclic shearing. Finally, the S–N curves could be determined for porous TBCs with any value of porosity.