Interfacial Fracture Toughness Measurement of Thick Ceramic Coatings by Indentation

Abstract

The basic requirement for the use of a ceramic coating is sufficient adhesion to its substrate. A measure of the adhesive properties of a coating is the interfacial fracture toughness. The test method applicable for interfacial fracture toughness measurements depends on the mechanical properties of the material system and the geometry of the test piece. In this work, indentation methods have been evaluated for the estimation of the fracture toughness of ceramic thermal barrier coatings on metallic substrates. Coatings of 100 to 300 µm thickness were applied by electron beam – physical vapour deposition. The performed test types were Vickers indentation at the interface of polished cross sections of the coating system and Rockwell indentation with a brale C indenter, penetrating the coating perpendicular to the surface. Both tests generate delamination, in which the delamination crack length corresponds to the interfacial fracture toughness. Fracture surfaces and cross sections of the fractured coatings were investigated by optical and scanning electron microscope. Determined fracture toughness values are discussed with respect to the loading conditions in the test and the fracture process – i.e. interaction between indenter and coating system and the crack propagation path.

Progress in Indentation Study of Materials via Both Experimental and Numerical Methods

“…In contrary, the hardness dramatically increased with decreasing residual indent depth for films indented by the Cube-corner indenter. Indentation tests were also used to evaluate adhesion strength of the thermal barrier coatings (TBCs) which were used to improve the performance and efficiency of advanced gas turbines [34][35][36][37][38]. Indentation tests were also used to evaluate adhesion strength of the thermal barrier coatings (TBCs) which were used to improve the performance and efficiency of advanced gas turbines [34][35][36][37][38].…”

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

“…However, several methods have been investigated and proposed. These include utiliz ing the ''pull-out technique" that is traditionally used to study fibers in reinforced ceramics or metals [9], four-point bend testing with a notched coating that initiates the delamination [10], and indentation techniques [5,[11][12][13]. The indentation technique shows great promise, since it requires only minimum sample preparation along with small quantities of material and is easy to perform.…”

“…Even though the technique proposed to evaluate the interfacial fracture toughness of TBCs shows great prom ise, some problems appeared when evaluating TBCs with thicker ceramic top coats and TBCs that have been sub jected to thermal aging [13]. An earlier experimental study [13] indicated that the technique developed in Ref.…”

“…An earlier experimental study [13] indicated that the technique developed in Ref. [5] pre dicts that a TBC that has been subjected to extended times at elevated temperatures would have higher interfacial frac ture toughness than as-processed samples [13].…”

“…[13] are due to the role of the microstructural response of the top coat, which included crushing and cracking in the top coat. We note that the system investigated by Bartsch et al [13] had a thicker top coat than was investigated in Ref. [5] (200 lm vs. 100 lm), which is beyond the range for what Ref.…”