On stresses induced in a thermal barrier coating due to indentation testing

On cracks and delaminations of thermal barrier coatings due to indentation testing: Experimental investigations

“…By measuring the size of the annular debonding cracks, we found that: (i) the as-coated samples resulted in larger debonding cracks than the aged samples; and (ii) the crack response could be divided into two regions: one response for ''low" maximum indentation force and another for ''high" maximum inden tation force. The latter is the main focus of a companion paper [16]. The first observation could be interpreted as the aged samples having more resistance to failure than the as-coated, which is at variance to the current under standing of TBCs, and is investigated in this paper as well as in a companion paper [16].…”

“…Thus, care must be taken when using indentation techniques to establish interfacial fracture toughness when the coating cannot be treated as ''thin" compared to the indentation. In the com panion paper to this study, numerical simulations to increase the understanding of indenting TBC systems are conducted [16] .…”

“…Moreover, it appears as the response can be divided into two cases: ''small" and ''large" maximum indentation force. The bifurcation in the response occurs around 175 N. This curious behavior is discussed in a companion paper, where the indentation is investigated through numerical simulations, and it is seen that the bifurcation between ''small" and ''large" indenta tion load is associated with the columnar structure [16]. The debonding path along the interface of the top and bond coats depends on the time-at-temperature and is influenced by local microstructural features.…”

“…Microstructural evolution of the TBC system due to thermal aging. For the as-coated system: (A) schematic illustrating gaps between columns along with a thin TGO; (B) micrograph of a near-surface region; and (C) micrograph of the interfacial region where the enlargement indicates a multiphase assembly in the TGO [16]. For the aged samples: (D) schematic illustrating narrowed gaps and rigid interconnections between the top coat columns due to sintering, and a two-layered TGO; (E) micrograph of a near-surface region after 200 h aging in air, and (F) SEM image of the TGO after 200 h aging at 1000 �C in air, showing two distinct layers: a porous layer adjacent to the top coat and a dense layer adjacent to the bond coat.…”

“…Since previous experimental results suggest an important interac tion between the indenter and the top coat on the size of deb onding cracks [13,15], we will in this paper focus on the micromechanical response of the ceramic top coat, including crushing and formation of cracks. In a companion paper, we conduct numerical simulations to elucidate the response seen from the experimental results presented here [16].…”