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 the thermal conductivity and mechanical properties of the TBC. In this study, the influence of the plasma spray process on the microstructure of the coating and deposition efficiency was investigated with the aim of achieving better knowledge and control of the process. Eight process parameters to control the plasma process were employed in a fractional factorial designed experiment involving 16 different thermal barrier coatings. The microstructure of the coatings, characterized by seven features, in particular those of cracks and pores, was studied by means of scanning electron microscopy (SEM) and image analysis (IA) and the extent of the different features were quantified. For each sprayed coating, the particle velocity and particle temperature were measured prior to impact, using the optical measure system DPV 2000. The four spray gun parameters controlling the plasma plume were found to each have a significant influence on the particle properties. The remaining four parameters did not affect the particle properties, but instead influenced the coating microstructure directly. Multiple linear regression was 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 substrate temperature to be the most important parameters influencing the coating microstructure. The influence of the different parameters and particle properties on the microstructure features varied, however.