Heat transfer analysis of the plasma spray deposition process

• Main Author: Wong, Henry Wing-Wo
• Language: English
• Published: 2009
• Online Access: http://hdl.handle.net/2429/6835

A novel approach has been used to analyse the flow of heat during the plasma spray coating process. The approach couples heat transfer processes occurring at the micro scale, within the coating, and at the macro scale, in the substrate, using a combination of finite-element and finite-difference based models. One of the key boundary conditions employed in the model has been determined from an inverse heat conduction analysis of data recorded from an array of thermocouples embedded into a copper disk while being sprayed with a stationary gun in the absence of powder deposition. The models have been used to determine the evolution of temperature in 8 wt% yttria partially stabilized zirconia coatings deposited onto AISI-1008 steel for the deposition times of 1, 2 and 3 seconds. The results indicate that the cooling rate of the material deposited initially is of the order of 10⁷ °C/s and that the first splat cools from its melting temperature to room temperature in approximately 10⁻³ s. The predictions from the model have been compared qualitatively with the distribution and the shape of pores found in the coating after subsequent metallographic examination. Different size and distribution of pores found at different locations of the zirconia based coating appear consistent with a qualitative understanding of the evolution of temperature in the coatings examined. Pores located at the coating / substrate interface appeared larger and were inter-connected. At the centre near the free surface of the coating, however, pores were closed and isolated. In addition, the same models have been applied for the plasma spray deposition of 17% Co-WC and CP titanium on AISI-1008 steel. The particle temperatures predicted using the model were compared with the temperatures measured in an independent study using a pyrometer-based measurements on the same plasma spray system. The results of the heat flow-based analysis were found to agree with the pyrometer-based measurements to within 10% for the case of 17% Co-WC and to within 16% for CP titanium.