Damage Evolution in Thermal Barrier Coatings with Thermal Cycling

Abstract

Thermal barrier coatings typically fail on cooling after prolonged thermal cycling or isothermal exposure. The mechanics of spalling requires that first a critical sized portion of the coating separates from the underlying material, then buckles and finally spalls away. The critical size for buckling depends on the thickness of the coating but is several millimeters for typical zirconia coatings 150 μm thick. As‐deposited coatings do not have interface separations but they form on thermal cycling as described in this work based on observations of coating cross‐sections combined with the stress redistribution in the thermally grown oxide imaged using a piezospectroscopic luminescence method. Analysis of the images reveals that small, isolated regions of damage initially form and then grow, linking up and coalescing to form percolating structures across the coating until the buckling condition is attained, the buckle extends and failure occurs by spallation. The piezospectroscopic imaging of the stresses in the thermally grown oxide formed by oxidation beneath thermal barrier coatings provides a form of “stress tomography” enabling the subcritical separations to be monitored.

A mechanistic understanding on rumpling of a NiCoCrAlY bond coat for thermal barrier coating applications

“…A possible reason for this phenomenon is that the TGO cannot undergo enough stress relaxation as surface rumpling of the bulk NiCoCrAlY alloys is small, the TGO stresses alleviate through spalling at a sufficiently high level of thickness [7]. Another possible reason could be embrittlement of the TGO/alloy interface and subsequent low interfacial strength caused by impurity segregation (e.g.…”

Influence of embedded MoSi2 particles on the high temperature thermal conductivity of SPS produced yttria-stabilised zirconia model thermal barrier coatings

“…The coated systems experience high stresses that develop due to the thermal expansion coefficients mismatch between the metal substrate and the ceramic TBC. Upon a sufficient number of cycles these stresses may result in the development of microcrack patterns in the TBC that coalesce and ultimately lead to failure of the topcoat and its adhesion promoting thermally grown oxide (TGO) layer [1][2][3][4]. For TBC systems manufactured by APS failure is known to occur within the porous YSZ top coat close to the TGO layer [3].…”

Power-law characteristics of damage and failure of ceramic coating systems under three-point bending

“…(3). When the controlling variable is close to the catastrophic point, the damage D increase rapidly and damage rate R tends infinite, i.e., (3), and neglecting the high-order term (higher second-order), the damage as a function of the controlling variable can be expressed as (4) shows that the damage D increases with increasing stress σ, and the evolution of the damage variable with the controlling variable obeys the power-law relation with the exponent of 0.5, which is similar to the power-law relation of 0.5 for the residual stress as the function of heat treatment time [21], differently, the residual stress decreases with increasing heating time [21], which is reasonable since the increased damage (crack) releases the strain energy and decreases the residual stress. Furthermore, the damage rate (sensitivity) of the ceramic coatings…”