Competition mechanism of interfacial cracks in thermal barrier coating system

Abstract

Abstract:The mechanism of competition between different interfacial cracks is investigated, specifically considering the cracks at top-coat/bond-coat and bond-coat/substrate interfaces in a thermal barrier coating system (TBCs). To assess the cracking process driven by mechanical loading, in-situ three-point bending tests were conducted on TBCs samples having different top coats (TC) and bond coats (BC). The competition between cracks at TC/BC and BC/substrate interfaces was observed for the first time, and then analyzed to elucidate the mechanism. Experimental and numerical results show that the differences in modulus ratio and thickness ratio of TC to BC are the main factors controlling the competition, which eventually leads to different fracture modes, i.e. when TC thickness is large or BC modulus is low, the coatings peel off from the TC/BC interface; while, with the decreasing TC thickness or increasing BC modulus, the delamination location changes to BC/substrate interface. A failure map based on the two ratios is numerically established, which agrees well with the experimental results. It is advised that the crack propagation path can be controlled by adjusting the combination of the two ratios to trigger both TC/BC and BC/substrate interfacial cracks in TBCs, thereby leading to more energy dissipation and better bending-resistance.

Thermal‐cycle dependent residual stress within the crack‐susceptible zone in thermal barrier coating system

“…The decrease in residual stress is mainly due to the nucleation and coalescence of microcracks in the crack-susceptible zone, as inferred from Figure 4. The standard deviation (60-75 MPa) of residual stress in the relaxation stage is much larger than that (15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25) in the stress development stage. This further confirms that the microcracks nucleate at different locations and their coalescence leads to the spallation.…”

“…The TBC specimens were finally cut into a rectangular shape (2.5 mm × 3.5 mm) and the cross‐section is schematically shown in Figure B. Detailed parameters used in thermal spraying can be found in a previous study …”

“…stress sensor) embedded in the crack-susceptible zone [Color figure can be viewed at wileyonlinelibrary.com] used in thermal spraying can be found in a previous study. 24…”

Experimental and Numerical Investigation on the Thermal and Mechanical Behaviours of Thermal Barrier coatings Exposed to CMAS Corrosion

“…The calcium-magnesium-alumino-silicate (CMAS) corrosion is believed to be one critical failure mechanisms which causes the open structure of TBC [9][10][11]. CMAS is a kind of mixed oxides usually coming from sand, runway debris, volcanic ash, air pollution, or fly ash [14].…”

Fatigue assessment of multilayer coatings using lock-in thermography

“…This will enable the assessment of fatigue performance of thin coatings (where replication approaches are less successful in tracking fatigue initiation and growth processes on the very thin electroplated coatings), and the improvement of damage assessment location in post-mortem observations. Such improvements help elucidate the mechanisms leading to the fatigue and fracture of the coatings [33,34], making possible the optimization and informed design of new fatigue resistant bearings. This is in contrast to the empirical process followed by using industry standard approaches such as the Sapphire test.…”