Influence of the mechanical properties of CoNiCrAlY under-coating on the high temperature fatigue life of YSZ thermal-barrier-coating system

Yttria-stabilized zirconia (YSZ) thermal-barrier-coatings (TBC) have been used for insulating and protecting the components from high temperature. However, the mechanical properties of the coatings are not well known, because the characterization of thin coatings is difficult. Consequently, how should we choose the mechanical properties, which is very important, is unsolved problem. In this paper, the effects of the mechanical properties of under-coatings, CoNiCrAlY, on the fatigue life of YSZ coating system were examined. First, the strength of three types of CoNiCrAlY coatings were examined by lateral compression of circular tube free-standing coating. The residual stresses of CoNiCrAlY coatings were also measured by X-ray diffraction method. Furthermore, the adhesive strengths of CoNiCrAlY coatings were measured by indentation method. Subsequently to the systematic understanding of mechanical properties of the CoNiCrAlY coatings, fatigue tests were carried out for the TBC systems at both room temperature and 893 K. The results indicated an improvement of the fatigue life because of the restriction of crack initiation into the substrate. It was found that the under-coating with proper mechanical properties could significantly extend the fatigue life of the TBC system.