Processing and characterization of plasma-sprayed ceramic coatings on steel substrate: Part I. On coating characteristic
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8/8/03
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Processing and Characterization of Plasma-Sprayed Ceramic Coatings on Steel Substrate: Part I. On Coating Characteristics S. DAS, P.P. BANDYOPADHYAY, T.K. BANDYOPADHYAY, S. GHOSH, and A.B. CHATTOPADHYAY This investigation envisages the processing of a series of plasma-sprayed coatings from a few commercially available and inexpensive powders, namely, alumina (commercial grade, Indian), plasmadissociated zircon (PDZ), zircon sand, and zircon-20 wt pct calcia. These powders do not belong to the so-called “plasma sprayable” grade, expensive powders. The microstructures and several properties of these coatings have been studied to evaluate their potential as thermal barrier and wearresistant coatings. With an appropriate choice of processing condition, a sound and adherent ceramic coating is achievable using such powders. In some coatings, a layer of yttria has been applied between the top and bond coats with an aim to improve its thermal barrier properties. Such a layer does not disrupt the interfacial continuity of the coatings. The powders have been found to undergo phase transformations during spraying, subsequent annealing, and also during tribological testing of the coatings. An understanding of such phase transformations is important for the interpretation of coating behavior during performance tests as wear-resistant and thermal barrier coatings. These responses are dealt with in Part II of this series of articles.
I. INTRODUCTION
AMONG various thermal spray techniques, plasma spraying has wide applications. High melting point materials such as carbides and oxides may be conveniently sprayed on to various substrates by the plasma-spraying technique.[1] During spraying, the molten droplets arrive at the substrate one at a time and undergo deformation to a pancake shape. On microscopic observation of the cross section of the coating, such “splat like” morphology can be observed.[2] Entrapped gases, unmelted particles, and voids are mixed with the particle splats. Their presence tells upon the coating properties, because they create points of stress concentration during in-service loading. Larger particles do not melt completely down to their core, and thus do not spread on impact. Thus, the process yields a porous coating with an irregular reticula of cracks running through it.[3] This is especially true for brittle materials.[4,5] The porosity adversely affects the wear property,[6] and both the cracks and pores allow the environment to attack the bond coat.[7] A high amount of porosity would allow a very quick oxygen ingress to the bond coat resulting in a failure by oxidation. A low porosity level, on the other hand, reduces the ability of the coating to accommodate stress. Therefore, a critical amount of porosity in the coatings must be achieved.[8] A close contact between the splat and the substrate created by the high pressure generated during droplet impingement contributes to the possibility of diffusion between S. DAS, Associate Professor, T.K. BANDYOPADHYAY, Graduate Studen
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