Strain dependent stiffness of plasma sprayed thermal barrier coatings

The strain dependency of the stiffness of bonded and free-standing thermal barrier coatings (TBCs) was analyzed using four point bending. The deformation in top and side face of the material was observed in-situ using optical and scanning electron microscopy. For free-standing TBCs, the strain of the tensile part above the neutral bending axis was larger than on the compressive part. Furthermore, the ratio of strain of tensile to compressive part increased with increasing applied strain and the position of the neutral axis sifted further to compressive side. The stiffness for free-standing TBC changed from 8 to 17 GPa with applied strain. For bonded TBCs, the average stiffness under tensile or compressive bending was determined considering residual stresses. The stiffness of the bonded TBCs increased from 12 to 38 GPa as the applied strain was increased from −0.75 to 0.11%. This strong strain dependency of the TBC stiffness can be associated with the large number of micro-cracks and pores. Finally, a unifying stress–strain curve is presented for strains from −0.75 to 0.23% obtained using results of the free-standing and bonded TBC.