Quality improvement of plasma sprayed chromia coatings by in situ dry ice processing
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Plasmaruiskutettujen kromioksidipinnoitteiden laadunparannus kuivajääpuhalluksella
Atmospheric plasma spraying is a commonly used process to deposit ceramic coatings for applications of wear and corrosion protection. Feedstock materials include for example aluminium oxide, titanium oxide and chromium oxide. Plasma sprayed chromium oxide coatings are widely used in for example anilox rolls, pump seals and wear rings for their good surface quality, high hardness and wear resistance. Chromium oxide is however challenging to spray due to its high melting point, low thermal conductivity and tendency to vaporize in high temperatures. The vaporization of chromium oxide during spraying creates extremely fine dust particles, which gather on the workpiece and are trapped inside the coating layers reducing the cohesion and mechanical properties of the coating. Dry ice blasting has been used in the field to improve the quality of chromium oxide and many other plasma sprayed coatings by keeping the surfaces clean and helping with thermal management. The use of dry ice blasting during spraying was investigated by plasma spraying chromium oxide coating at TUT with two different commercial dry ice blasters attached to the spraying robot. Several parameters were tested and temperature monitoring was implemented. Metallographic specimens were prepared and analysed by SEM. Hardness, adhesion, gas permeability and wear tests were also conducted. It was found that dry ice blasting modifies the temperature history of the substrate and coating dramatically having unexpected effects. Excessive cooling lessened splat to splat bonding lowering cohesion and wear resistance but adjusting the spraying parameters hotter eliminated some of the adverse effects. There were also great differences in different blaster models related to the size of the particles exiting the nozzle. While the other blaster sprayed only small dry ice dust that mainly cooled the substrate, the other sprayed larger pellets with greater kinetic energy having a much more positive effect on coating cohesion increasing wear resistance compared to non-dry ice blasted samples.