MODELING OF THERMAL HISTORY OF SPLATS LAYERING DURING COATING FORMATION

Adhesion/cohesion and residual stresses of plasma sprayed coatings are closely linked to their thermal history. Coatings are built by layering splats. It has recently been shown for alumina and zirconia splats, that their shape, microstructure and adhesion to substrate or previously deposited layers depend strongly on the mean temperature (as measured for example with a slow (10 Hz) IR pyrometer) of the surface on which particles impact. However questions arise about the real meaning of this mean surface temperature ? Substrate and coating are heated by the heat flux of the plume of the spraying plasma jet and by the impacting particles releasing, upon the resulting splats cooling, their latent heat of solidification and their heat content. These phenomena are transient with very different time scales: ranging between 1 μs and hundreds of seconds. In order to understand better what is going on a simplified ID model has been developed to calculate the temperature-time history of alumina and zirconia layering splats. The solidification has been assumed to occur at melting temperature. The results of the model for typical spraying conditions, from thin passes (3 to 15 μm in thickness) to thick ones (up to 180 μm) with different air cooling flow rates to reduce the heat flux from the plasma plume are presented. A tentative is made to correlate the calculated temperature-time histories to the obtained coating microstructures.