| Summary: | 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.
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