Life prediction and failure mechanisms for thermal barrier coatings

A life prediction model has been developed based on experimental observations of the fracture paths and failure modes for two layer thermal barrier coatings (TBCs) during cyclic thermal oxidation. It was experimentally observed that the spalling of these coatings took place primarily within the MCrAlY bond coat oxidation products or at the interface between the bond coat oxidation products and the partially stabilized zirconia (PSZ) TBC or the MCrAlY bond coat. It was also observed that the fracture within these oxidation products occurred with the Ni(Cr,Al){dollar}sb2{dollar}O{dollar}sb4{dollar} spinel or Cr/Ni rich oxides. It was found that Al{dollar}sb2{dollar}O{dollar}sb3{dollar} developed first followed by the formation of the (Al,Cr){dollar}sb2{dollar}O{dollar}sb3{dollar} solid solution and the Ni(Cr,Al){dollar}sb2{dollar}O{dollar}sb4{dollar}. It was therefore concluded that the formation of these oxides was a "life-limiting event" for TBCs and defines the time required for failure of the TBCs by spalling.; Based on this observation a finite difference computer model was developed to predict the formation of the Ni(Cr,Al){dollar}sb2{dollar}O{dollar}sb4{dollar}, Cr{dollar}sb2{dollar}O{dollar}sb3{dollar} and NiO. These oxides were assumed to form just after the {dollar}beta{dollar}-NiAl phase has been disappeared in the MCrAlY bond coat. The predicted lifetimes were found to be in good agreement with the experimental lifetimes of the TBCs. The computer experiments were carried out to determine the significance of the parameters affecting TBC life. Computer experiments showed that the life of the TBC system increased with the MCrAlY bond coat thicknesses and aluminum content, while the life of TBC system decreased with D{dollar}sb{lcub}rm Al{rcub}{dollar} and K{dollar}sb{lcub}rm p{rcub}{dollar}. The bond coat thickness and the aluminum content were found to be more significant to TBC life than D{dollar}sb{lcub}rm Al{rcub}{dollar} and K{dollar}sb{lcub}rm p{rcub}{dollar}.; This model may be used not only to predict TBC life in service but also to determine the effects of variations on the life of the TBC. This life prediction model will not apply for cases where the failure of the TBC occurs within the PSZ ceramic coating.