Processing and characterization of plasma-sprayed ceramic coatings on steel substrate: Part I. On coating characteristics

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), plasma-dissociated 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 wear-resistant 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.