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

TL;DR: In this article, the authors investigated the experimental results by numerical simulations incorporating the material microstructure and found that indentation testing of multilayered coated structures might not induce the delamination in the overall weakest interface and therefore the test results must be evaluated with care.

Abstract: Instrumented indentation has been suggested as a method to determine interfacial fracture toughness of thermal barrier coatings. However, in a previous experimental study we showed that the results are ambiguous. In this work, we investigate the experimental results by numerical simulations incorporating the material microstructure. In the numerical simulations, based on finite element analyses, the stress fields that are associated with the loading and unloading of the indenter are investigated. By comparing these stress fields to the damage observed in the experimental study, including crack path and interfacial delaminations, we explain key findings from the experimental observations. Our results suggest that indentation testing of multilayered coated structures might not induce the delamination in the overall weakest interface and therefore the test results must be evaluated with care.

Summary

1. Introduction

• Thermal barrier coatings (TBes) are multilayered coatings that are frequently used in gas turbine applications to protect structural components from the intrinsic high temperatures.
• Even though there are several possible scenarios that eventually can lead to the failure of a TBe.
• The challenges associated with designing and tes ting TBCs comes from the multilayered structure of the coating, where the properties evolve as the system is used.
• The TGO commonly also has other oxidation prod ucts that may affect the overall interfacial st rength.
• In the following, the authors first summarize the experimental results, before discussing the finite element models and the results.

2.1. Specimens and experimental procedures

• Flat specimens of IN 625 and a limited number of CMSX-4 were coated by electron beam physical vapor deposition (EB-PVD), first with a NiCoCrAlY bond coat (100 lm) followed by a partially stabi lized YSZ (7–8 wt% yttria, 280 lm).
• The samples were kept at high temperature for 23 h and at room temperature for 1 h, until the specified ‘‘time-at-temperature” was reached.
• Spontaneous spallation occurred in the samples aged to 400 h; consequently, these were not used in the indentation testing.
• During the indentation testing, the indentation displacement and force were recorded continuously.
• Based on these curves, it appears that there is one type of response for lower indentation forces and another for higher indentation forces, where the lower maximum indentation forces result in a higher slope (of the delamination–indentation force curve) than for the higher maximum indentation forces.

2.2. Experimental observations

• The heat treatment of the samples causes changes in the micro structures, including sintering of the YSZ and growth of the TGO, as illustrated in Fig. 1 [6].
• Aging of the system is simulated by changing three classes of parameters: (i) Increasing the width of the columns in the top coat and decreasing the distance (ICS) between the columns.
• The selected geometry is presented in Table 1. (ii) Increasing the thickness of the TGO, combined with decreas ing the thickness of bond coat.
• Results and discussion of-plane” stress (associated with mode I at the interface), r22, shown in Fig. 6, and ‘‘shear stress” (associated with mode II at As mentioned previously, the authors will conduct a qualitative assess- the interface), r12, as shown in Fig.
• For the cases of higher maximum indentation force, the unloaded stress state shows that the stress level decreases in the interface under the indenter and vanishes at the higher indentation forces (Fig. 8E and F).

5. Concluding remarks

• The response from using Rockwell indentation as a means of establishing the interfacial fracture toughness in thermal barrier coatings (TBCs) was explored by numerical simulations.
• In addition, for a given top coat column width, different maximum indentation forces (or depths) lead to different bending deforma tion of top coat columns, thus causing distinct influence zones via columnar interactions.
• Thus, the authors believe that the experimentally observed discrepancy is due to the toughness change of the TGO-system due to ageing.
• This model did not include the crack propagation and was therefore not able to capture this behavior.

Figures

Table 1 Geometry of the columnar model (ICS, intercolumnar spacing, Fig. 4)

Fig. 11. The interfacial stresses for maximum indentation forces of 100, 200, and 500 N: (A) tensile stress and (B) shear stress for the homogeneous top coat; (C) tensile stress and (D) shear stress for the columnar coat. TGO–bond coat interface, 100 h aged conditions assumed (Tables 1 and 2).

Fig. 10. The interfacial stresses (A) tensile stress and (B) shear stress, for 100 N maximum indentation force for the three interfaces. Columnar top coat, 100 h aged conditions assumed (Tables 1 and 2).

Fig. 6. Out-of-plane stresses (r22) for homogeneous top coat at maximum load (left column) and after unloading (right column), for maximum indentation forces of 100, 200, and 500 N (top to bottom). Hundred hours of aged conditions assumed (Tables 1 and 2).

Fig. 9. Shear stresses (r12) for columnar top coat at maximum load (left column) and after unloading (right column), for maximum indentation forces of 100, 200, and 500 N (top to bottom). Hundred hours of aged conditions assumed (Tables 1 and 2).

Fig. 15. The interfacial stresses for as-coated, 50 h and 100 h aged specimens. Maximum indentation force 100 N (A) tensile stress and (B) shear stress; and for 500 N (C) tensile stress and (D) shear stress. Columnar coat, TGO–bond coat interface.

Fig. 1. Schematic and SEM images of the thermal barrier system, showing as-coated samples and samples aged for 200 h at 1000 �C in air. The as-coated sample shows (B) the intercolumnar spacing (ICS) between YSZ columns near the top coat surface and (C) the intermixed, porous (preexisting) TGO. The aged samples show (D) narrowed ICS and rigid connections between the YSZ columns and (E) the bi-layered structure of the TGO. For both as-coated and aged samples, the top coat closest to the TGO does not show expressed ICS between columns.

Fig. 2. Schematic of the main pattern after indentation, including the kink-bands. The extended annular delamination crack tends to propagate in the TGO and TGO–bond coat interface for as-coated samples but does not reach the bond coat interface for the aged samples.

Fig. 14. For the case of indentation force 500 N, out-of-plane stresses (r22) for columnar top coat at maximum load (left column) and after unloading (right column). Ascoated, 50 h, and 100 h aged (top to bottom).

Fig. 7. Shear stresses (r12) for homogeneous top coat at maximum load (left column) and after unloading (right column), for maximum indentation forces of 100, 200, and 500 N (top to bottom). Hundred hours of aged conditions assumed (Tables 1 and 2).

Fig. 12. Peak interfacial tensile stress as a function of maximum indentation force. 100 h aged conditions assumed (Tables 1 and 2). The stars symbolizes calculated values, the lines are ‘‘guide-for-the-eye.”

Fig. 13. For the case of indentation force 100 N, out-of-plane stresses (r22) for columnar top coat at maximum load (left column) and after unloading (right column). As-coated, 50 h, and 100 h aged (top to bottom).

Fig. 8. Out-of-plane stresses (r22) for columnar top coat at maximum load (left column) and after unloading (right column), for maximum indentation forces of 100, 200, and 500 N (top to bottom). Hundred hours of aged conditions assumed (Tables 1 and 2).

Fig. 4. Schematic of the axi-symmetric model with columnar top coat including the intercolumnar spacing (ICS), d. The radius of the indenter tip is included in the model. (The standard Rockwell brale C indenter as used in the test has a tip radius of 0.2 mm.)

Fig. 5. Force-displacement curves from experiments and numerical simulation of the columnar structure: (A) as-coated, (B) 50 h aged, (C) 100 h aged.

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