Processing and characterization of plasma-sprayed ceramic coatings on steel substrate: Part II. On coating performance
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8/11/03
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Processing and Characterization of Plasma-Sprayed Ceramic Coatings on Steel Substrate: Part II. On Coating Performance S. DAS, P.P. BANDYOPADHYAY, S. GHOSH, T.K. BANDYOPADHYAY, and A.B. CHATTOPADHYAY This investigation envisages the performance evaluation of a series of plasma-sprayed coatings from a few inexpensive powders, namely, alumina, plasma dissociated zircon (PDZ), zircon sand, and zircon-20 wt pct calcia. The hardness and adhesion (bond) strength of the coatings have been evaluated. All coatings offer an attractive thermal fatigue property. An added layer of yttria between the top and bond coats improves the thermal fatigue resistance appreciably. The wear performance of these coatings has also been evaluated using a “pin-on-disc” type wear testing setup. The Indian alumina coating offers a superior wear resistance and thermal fatigue property compared to an imported alumina coating. The coatings based on zirconia-silica offer an attractive thermal fatigue resistance along with an acceptable wear resistance. The latter improves considerably upon annealing of the coatings because of the removal of the residual stresses accumulated during deposition. Among these zirconia-silica-based coatings, the zircon-20 wt pct calcia is the most promising. The finishability of these coatings has been studied using a surface grinding setup equipped with a diamond wheel. The cutting forces and the surface roughness of the ground surface have been measured. The coatings have been found to possess a reasonably good grindability.
I. INTRODUCTION
PLASMA spraying is a versatile and technologically sophisticated thermal spraying technique. The process can be applied to coat a variety of complicated shapes and sizes using metallic, ceramic, or polymeric consumables. The production rate of the process is very high and the coating adhesion is also adequate.[1] Because the process is almost material independent, it has a very wide range of applicability, e.g., as thermal barrier coatings, wear resistant coatings, etc. Zirconia is a conventional thermal barrier coating material.[2] Alumina, titania, and zirconia are examples of wear-resistant materials.[3,4,5] Thermal barrier coatings are applied to protect the engineering components from the high-temperature environment.[6–10] Typical examples of thermal barrier coating materials are partially stabilized zirconia (PSZ),[6] dicalcium silicate,[7] calcia-silica-zirconia,[8] and calcium titanate.[7] The capability of thermal barrier coatings is normally tested by thermal cycling, where test specimens are exposed to a high-temperature environment for some time and then suddenly cooled. The cycle is repeated many times. Failure of the coatings occurs owing to stresses developed at the interface of the metallic bond coat and the ceramic top coat because their thermal expansion coefficients differ greatly.[11] The other important reason for failure of the thermal barrier coatings is the oxidation of the bond coat. During S. DAS, Associate Professor, T.K. BAN
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