Influence of particle in-flight characteristics on the microstructure of atmospheric plasma sprayed yttria stabilized ZrO2

The atmospheric plasma spraying of a yttria stabilized ZrO2 top-layer of a thermal barrier coating (TBC) produces a complex microstructure consisting of a wide variety of cracks and pores. These voids are known to influence t he thermal conductivity and mechanical properties of the TBC. In this study , the influence of the plasma spray process on the microstructure of the co ating and deposition efficiency was investigated with the aim of achieving better knowledge and control of the process. Eight process parameters to co ntrol the plasma process were employed in a fractional factorial designed e xperiment involving 16 different thermal barrier coatings. The microstructu re of the coatings, characterized by seven features, in particular those of cracks and pores, was studied by means of scanning electron microscopy (SE M) and image analysis (IA) and the extent of the different features were qu antified. For each sprayed coating, the particle velocity and particle temp erature were measured prior to impact, using the optical measure system DPV 2000. The four spray gun parameters controlling the plasma plume were foun d to each have a significant influence on the particle properties. The rema ining four parameters did not affect the particle properties, but instead i nfluenced the coating microstructure directly. Multiple linear regression w as used to find models describing how the particle properties and the other process parameters were related to the coating microstructure. The results showed particle velocity, particle temperature, spraying angle and substra te temperature to be the most important parameters influencing the coating microstructure. The influence of the different parameters and particle prop erties on the microstructure features varied, however. (C) 2001 Elsevier Sc ience B.V. All rights reserved.