Post deposition treatment of thermal sprayed coatings

• Main Author: John, George
• Language: English
• Published: 2009
• Online Access: http://hdl.handle.net/2429/4628

Thermal sprayed coatings are frequently used in corrosive environments, even when their major purpose is to provide wear or thermal resistance, rather than corrosion resistance. This includes Thermal Barrier Coatings (TBC), where high porosity is a desired feature to give good thermal protection. Porosity also provides increased strain tolerance of the coating. However, as this proves to be a limiting factor in the corrosion protection, a trade off is involved. This is because the interconnected porosity in TBCs allows the corrosive media to reach the coating-substrate interface, which eventually leads to delamination of the coatings. The current techniques to reduce coating permeability include polymer impregnation and laser glazing. Polymer impregnation into the pores is difficult and it severely limits the high temperature properties of the parent coating. Laser glazing produces segmented cracks in the coating which are useful in providing strain tolerance, but accelerate the rate of bond coat oxidation. This work addresses the problem of permeability of TBCs which is responsible for the poor corrosion protection offered by these coatings. The coatings studied included yttriastabilized zirconia and alumina TBCs. A simple infiltration technique has been proposed using sol-gel ceramic precursors. The precursors included metal alkoxides which decompose at relatively low temperatures to their respective oxides. It has been proven that these precursors effectively penetrate into the pores and are beneficial in drastically reducing the coating permeability to gases. Electrochemical tests were carried out in 3.0 wt.% NaCl solution to study the effectiveness of the sealant in reducing the coating permeability. These potentiodynamic tests as well as gas permeability tests show a considerable decrease in interconnected porosity with sol-gel modification of the coatings. Burner rig tests show an increase in sealed coating life time under thermo-mechanical fatigue conditions. The peel adhesion tests confirm an increase in the adhesion values with such a post deposition treatment. It is concluded that the advantages of sol-gel route to ceramics have been successfully utilized to post-treat porous thermal sprayed coatings in an attempt to reduce their permeability and to increase the life expectancy under corrosive and/or thermo-mechanical fatigue conditions.