A critical assessment of in-flight particle state during plasma spraying of YSZ and its implications on coating properties and process reliability

Air plasma spray is inherently complex due to the deviation from equilibrium conditions, three dimensional nature, multitude of interrelated (controllable) parameters and (uncontrollable) variables involved, and stochastic variability at different stages. The resultant coatings are complex due to the layered high defect density microstructure. Despite the widespread use and commercial success for decades in earthmoving, automotive, aerospace and power generation industries, plasma spray has not been completely understood and prime reliance for critical applications such as thermal barrier coatings on gas turbines are yet to be accomplished.;This dissertation is aimed at understanding the in-flight particle state of the plasma spray process towards designing coatings and achieving coating reliability with the aid of noncontact in-flight particle and spray stream sensors. Key issues such as the phenomena of optimum particle injection and the definition of spray stream using particle state are investigated. Few strategies to modify the microstructure and properties of Yttria Stabilized Zirconia coatings are examined systematically using the framework of process maps. An approach to design process window based on design relevant coating properties is presented. Options to control the process for enhanced reproducibility and reliability are examined and the resultant variability is evaluated systematically at the different stages in the process. The 3D variability due to the difference in plasma characteristics has been critically examined by investigating splats collected from the entire spray footprint.