TGO growth behaviour in TBCs with APS and HVOF bond coats

The growth of thermally grown oxide (TGO) layers and their influence on crack formation were studied for two thermal barrier coating (TBC) systems with CoNiCrAlY bond coats produced by (i) air plasma spray (APS) and (ii) high-velocity oxy-fuel (HVOF) techniques. All samples received a vacuum heat treatment and were subsequently subjected to thermal cycling in air. The TGOs were predominantly comprised of layered alumina, along with some oxide clusters of chromia, spinel and nickel oxide. However, after extended oxidation, the alumina layer formed in the APS-CoNiCrAlY bond coat transformed to chromia/spinel, while that formed in the HVOF-CoNiCrAlY bond coat remained stable. TGO thickening in the APS-CoNiCrAlY bond coat generally exhibited a three-stage growth behavior, which resembles a high temperature creep curve, whereas growth of the alumina layer in the HVOF-CoNiCrAlY bond coat showed an extended steady-state stage. Crack propagation in these two TBCs was found to be related to the growth and coalescence of oxide-induced cracking, connecting with pre-existing discontinuities in the topcoat. Hence, crack propagation during thermal cycling appeared to be controlled by TGO growth.