A sintering model for plasma-sprayed zirconia TBCs. Part I: Free-standing coatings

Abstract

A sintering model is presented for prediction of changes in the microstructure and dimensions of free-standing, plasma-sprayed (PS) thermal barrier coatings (TBCs). It is based on the variational principle. It incorporates the main microstructural features of PS TBCs and simulates the effects of surface diffusion, grain boundary diffusion and grain growth. The model is validated by comparison with experimental data for shrinkage, surface area reduction and porosity reduction. Predicted microstructural changes are also used as input data for a previously developed thermal conductivity model. Good agreement is observed between prediction and measurement for all these characteristics. The model allows separation of the effects of coating microstructure and material properties, and captures the coupling between densifying and non-densifying mechanisms. A sensitivity analysis is presented, which highlights the importance of the initial pore architecture. Predictions indicate that the microstructural changes which give rise to (undesirable) increases in thermal conductivity and stiffness are very sensitive to surface diffusion.

Heat Transfer Through Plasma-Sprayed Thermal Barrier Coatings in Gas Turbines: A Review of Recent Work

“…The spraying conditions, and coating microstructures, were similar to those reported elsewhere [31,[75][76][77]. The predictions shown were obtained using input data, and predictions from the sintering model, shown in Table I.…”

“…In particular, Cipitria et al [77,85] have recently published a model for sintering of PS YSZ TBCs, including simulation of the effects of attachment to a substrate [85]. The basic geometry of the system, which is illustrated in Fig.7, is similar to that used for simulation of heat transfer [15], although the model actually incorporates different domain structures for growth of the inter-splat contact area and for healing of the intra-splat microcracks.…”

“…However, it should be noted that it arises solely from grain boundary diffusion, since surface diffusion generates no volume change and lattice diffusion is negligible in these TBCs over the temperature range of interest. In fact, it is in many ways the surface diffusion that has the predominant effect during these heat treatments [77], particularly concerning growth of inter-splat "bridges". There are measurable parameters that are sensitive to surface diffusion, such as the specific surface area, and comparisons between predicted and measured values of this also show good agreement [77,85].…”

“…The basic geometry of the system, which is illustrated in Fig.7, is similar to that used for simulation of heat transfer [15], although the model actually incorporates different domain structures for growth of the inter-splat contact area and for healing of the intra-splat microcracks. A compiled version of the code for implementation of the model is available for downloading [77]. Fig.8 shows predicted and measured dimensional changes taking place in free-standing PS YSZ TBCs during heat treatments.…”

Thermal conductivity and thermal cyclic fatigue of multilayered Gd2Zr2O7/YSZ thermal barrier coatings processed by suspension plasma spray

“…Yttria stabilized zirconia (YSZ) is the standard top coat material used in thermal barrier coatings. But it undergoes significant sintering above 1200 º C, which is not desirable [4]. Furthermore, YSZ is susceptible to CMAS infiltration, which results in loss of strain tolerance and change in near surface mechanical properties leading to early coating spallation [5][6][7][8][9][10][11][12][13].…”