The displacement of the thermally grown oxide in thermal barrier systems upon temperature cycling

Evolution of surface morphology of thermo-mechanically cycled NiCoCrAlY bond coats

“…It does not matter if the non-elastic strain arrives as plastic strain or as creep strain. Creep by itself does not drive morphological instabilities [3]. We will later see that the morphological instabilities are governed by the high tem perature yield properties, whereas the properties below about 700 • C do not influence the surface instabilities.…”

“…Morphological instabilities are governed by the state of stress in the bond coat in combination with the inelastic strain of the bond coat at high temperatures, since the inelasticity (plasticity or creep) in the bond coat allows the TGO to relax and deform out of plane -into the bond coat -for each thermal cycle. The primary function of the growth stress is to differentiate between isothermal and thermal cyclic exposure: if the growth stress is assumed to be infinite (i.e., the TGO is fully elastic) isothermal conditions will result in large amplitude changes as well [3].…”

“…based on rate independent elastic-plastic properties, developed for previous studies is adopted [2,3,21,22], utilizing ABAQUS [23].…”

“…Thus, we will study the effect of the high temperature strength on the morphological instability by varying the value of the high temperature (time independent) yield strength in the calculations. 3 For morphological instabil ities to develop, accumulation of non-elastic strain in the bond coat is important. It does not matter if the non-elastic strain arrives as plastic strain or as creep strain.…”

“…Both lengthening and thickening components of the TGO are imposed, where the lengthening component is associated with the growth strain. The accumulation of lengthening growth strain is the key param eter driving the system [2,3,22]. For each calculation, we will assume constant growth strain, since we only simulate a fraction of the total number of cycles, thus assuming that the slow down in oxidation-rate is small within the time frame consid ered.…”