Microstructural aspects of the sintering of thermal barrier coatings

Densification of EB-PVD yttria-stabilized zirconia (YSZ) coatings occurs in two stages. Initially, the feathery morphology of the individual columns of the as-deposited coatings smoothens with the concurrent development of surface undulations. Then, some of these undulations on adjacent columns grow in amplitude until impinging to span the gap between the columns and form rows of local necks along the columns. The necks motivate sintering together of adjacent columns. The formation of necks and the overall response of the coating to this sintering process, however, depend on the roughness of the alloy surface. If the surface on which the coating is deposited is flat the columns are straight and parallel. Adjacent columns sinter together and the columnar microstructure is preserved with no cracking and the coating can withstand exposure to 1400 °C for at least 350 h with no transformation to monoclinic zirconia occurring. If the substrate is not flat, or develops roughness on thermal cycling, necks form and clusters of individual columns sinter together to form large-scale dense regions with larger gaps in between. The pattern of the gaps appears to reproduce the roughness of the underlying bond-coat alloy. The origin of this different behavior is attributed to the local undulations in the substrate surface causing individual columns to converge, narrowing the gap between them.